Username length issue with SMTP in Azure Communication Services

Question

Username length issue with SMTP in Azure Communication Services

Jonny Sharp 110

Hi,

I'm testing "Email SMTP as a Service" that is in preview within "Azure Communication Services". I was so excited to hear about it because we've just done an Exchange migration to O365, and this feature will help mop up the remaining SMTP client apps without having to deploy any SMTP relay infrastructure.
It was very easy to set up and get working for 2 test domains.... BUT...

The instructions state that a Service Principal must be used as the user. And with this, the username must be in the following format:
<Azure Communication Services Resource name>|<Entra Application ID>|<Entra Tenant ID>

This makes for a super long username. Mine was 94 characters! It worked in one SMTP client software but others I've tested have character length limits such as 60 characters.

The 2 pipes (or dots), App ID and Tenant ID guids mean that the username has 74 characters before the ACS resource name prefix is added!

I thought it might work with a UPN as username, so I added a user to the IAM Role Assignment, but that didn't work, returning error code "535 5.7.3 Authentication unsuccessful".

Is there a workaround? Because without one, the use of this SMTP feature will be severely limited.

Thanks,

Jonny S

brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-12-11T03:40:35.8966667+00:00

@Jonny Sharp Hi, thank you for reaching out. As you mentioned, this feature is in preview state currently. This means that we do not recommend the feature for production use as features are still being matured out into their GA state and there might be known bugs.

With that being said, let me reach out to the team and see if this is something they are aware of. I hope to reply back by mid-week.
brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-12-15T03:06:23.6033333+00:00

@Jonny Sharp I have not forgotten about your ask. I am still in discussion with the product group. Please give me a little more time. Thanks
Dag Efjestad 0 Reputation points

2024-02-01T13:44:26.39+00:00

Did you do anything with this?
Matt Mauchley 5 Reputation points

2024-02-06T22:53:43.77+00:00

We want information on this as well. Want to move our SMTP apps to Azure but most only support 50 character limit so it won't work as currently architected.
Ashley - Tetrabyte 25 Reputation points

2024-04-29T22:22:24.42+00:00

Just wanted to raise that I think i'm facing similar issues using CMailGUI 0.04 as a test platform

UPDATE: May 2025

For a full solution here is a quick fire guide on the steps:

Create an Azure Communication Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/create-communication-resource?tabs=windows&pivots=platform-azp

Create an Azure Communication Email Service (yes both are needed) https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/create-email-communication-resource?pivots=platform-azp

Add your Custom Domain to the Email Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/add-custom-verified-domains?pivots=platform-azp

Link that to the Communication Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/connect-email-communication-resource?pivots=azure-portal

Create and Register a App Registration Create a Secret and save the value https://learn.microsoft.com/en-us/entra/identity-platform/howto-create-service-principal-portal#option-3-create-a-new-client-secret

Add the Application to the Azure Communication Service (Not the Resource Group or Subscription, must be direct! on the Communication Service itself. Also not the email service) Then create an SMTP Username https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/smtp-authentication?tabs=built-in-role

Send an email:

-Server: smtp.azurecomm.net

-Port: Port 587 (recommended) or port 25

-TLS / StartTLS: Enabled

-Username: [The SMTP username you created in step 6]

-Password: [The Secret 'Value' you created in Step 5]

And your DONE! On a quick run this setup took 8 mins to do the whole setup.
Veselý, Ondřej 5 Reputation points

2024-05-28T12:08:35.93+00:00

I have exactly the same issue in UniFlow, Canon printers and in Navision(basically all my SMTP use cases).

I would assume that the length of the username AND password is going to be a major issue pretty much in majority of the applications where this could be used.
Mischa Rohleder 0 Reputation points

2024-05-29T13:36:28.4033333+00:00

I am having the same problem.. Is there any update?

We are cut at 64 characters
Thomas Oeser 0 Reputation points

2024-07-23T15:51:05.92+00:00

@brtrach-MSFT , any news regarding the character length issue for the username? We have some large enterprise customers willing to test the Azure Communication services relay functionality but this is a blocker for them. Especially printers have a character length limit of 64 characters for username field but as others already mentioned other applications have similar restrictions. This needs to be addressed to make this product usable.
Lorenzo Cernecca 6 Reputation points

2024-08-05T06:32:12.1566667+00:00

I have similar needs looking to use ACS as a SMTP solution for devices that don't support modern authentication. An example is a Dell iDRAC which appears to have a 50 character username limit.

If I only include the <Entra Application ID>|<Entra Tenant ID> component of the ACS username I am seeing 75 characters which makes it useless for this purpose. As I created a descriptive <Azure Communication Services Resource name> the combined total of my username came to 111 humorous characters.
Joe Widi 25 Reputation points

2024-09-05T20:36:03.94+00:00

I have this same issue; the username is entirely too long for even a brand-new app we're installing. We are actually stuck at a 50-character limit.
Brian House 0 Reputation points

2024-10-04T19:00:08.95+00:00

@brtrach-MSFT Did this user ever give you details to pursue this with the product group? I just ran into this exact issue today and will gladly relay my experience
Frank E 0 Reputation points

2024-10-07T18:14:32.9533333+00:00

@brtrach-MSFT I have the same issue. Can you please give us an update?
Frank E 0 Reputation points

2024-10-08T08:54:07.1833333+00:00

Is there any update on this? It's really annoying. Now i have to use a third party service.
Patrick Helm 5 Reputation points

2024-10-17T05:44:45.9466667+00:00

I'm facing a Kyocera vendor SMTP 64 character limit for username as well. :( All the effort to get the things working, to find this the blocker.
Alex Pettigrew 5 Reputation points

2024-11-04T22:50:08.59+00:00

If it eeks anyone below the limit their are facing in their application, you are able to remove the dashes in the app/tenant ID. But I am also looking for this to be fixed to be able to reliably use this product. Most legacy apps requiring smtp auth were simply not built with expectations of usernames this long.
Nebhuth, Garry 0 Reputation points

2024-11-20T15:39:56.49+00:00

next try..any updates ?
Link Martin 5 Reputation points

2024-12-11T14:14:10.6233333+00:00

@brtrach-MSFT Hi, is there any chance that the short usernames will come soon. We want to switch our MFP to this services, but we can´t because the usernames are to long.
Anne 10 Reputation points

2025-03-11T15:13:07.0366667+00:00

Does anyone know if Microsoft enabled the custom character limit for username?
de la Mar, Joeri 5 Reputation points

2025-03-12T12:22:35.9566667+00:00

Our organization also needs this, to be able to adopt this service properly. If this functionality will not be offered, ACS Email is a no-go.
Rick Carlson 10 Reputation points

2025-04-25T00:06:16.79+00:00

Has there been any movement from MS on allowing shorter usernames? I just went through the painstaking multi-day process of getting ACS working correctly, only to be unable to use it with 99.9% of our STMP clients. This is highly frustrating, as many of the services with this ~60 char username limit are from MS themselves!
Link Martin 5 Reputation points

2025-04-25T05:10:32.65+00:00

We did not recived any news about shorter Usernames. We have the same problem with the most of our MFPs and our ERP Systems. Only some are able to do oauth. The others normaly do not allow long usernames.

I cannot understand why MS is switching off the service in Exchange without providing another solutions whoch works.
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
David Wilson 0 Reputation points

2025-05-20T16:10:49.5466667+00:00

This is Nice! I was on support about this and they mentioned they were going to add the feature.

It does stink though that our xerox copiers don't take passwords larger than 31 characters. this makes it hard for me to use this smtp relay since a lot of what we use relay for is Xerox copiers. I think Microsoft's generated passwords are too large for this. if anyone has any workarounds I would love to know.
Emmitt Houston 0 Reputation points

2025-05-20T17:12:18.22+00:00

Depending on your licensing and need High Volume Email (HVE) may be an option for you. I have been working for a client that uses a Konica scan to email for mostly internal recipients.

5 answers

Your answer

brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-12-11T03:40:35.8966667+00:00

@Jonny Sharp Hi, thank you for reaching out. As you mentioned, this feature is in preview state currently. This means that we do not recommend the feature for production use as features are still being matured out into their GA state and there might be known bugs.

With that being said, let me reach out to the team and see if this is something they are aware of. I hope to reply back by mid-week.
brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2023-12-15T03:06:23.6033333+00:00

@Jonny Sharp I have not forgotten about your ask. I am still in discussion with the product group. Please give me a little more time. Thanks
Dag Efjestad 0 Reputation points

2024-02-01T13:44:26.39+00:00

Did you do anything with this?
Matt Mauchley 5 Reputation points

2024-02-06T22:53:43.77+00:00

We want information on this as well. Want to move our SMTP apps to Azure but most only support 50 character limit so it won't work as currently architected.
Ashley - Tetrabyte 25 Reputation points

2024-04-29T22:22:24.42+00:00

Just wanted to raise that I think i'm facing similar issues using CMailGUI 0.04 as a test platform

UPDATE: May 2025

For a full solution here is a quick fire guide on the steps:

Create an Azure Communication Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/create-communication-resource?tabs=windows&pivots=platform-azp

Create an Azure Communication Email Service (yes both are needed) https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/create-email-communication-resource?pivots=platform-azp

Add your Custom Domain to the Email Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/add-custom-verified-domains?pivots=platform-azp

Link that to the Communication Service https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/connect-email-communication-resource?pivots=azure-portal

Create and Register a App Registration Create a Secret and save the value https://learn.microsoft.com/en-us/entra/identity-platform/howto-create-service-principal-portal#option-3-create-a-new-client-secret

Add the Application to the Azure Communication Service (Not the Resource Group or Subscription, must be direct! on the Communication Service itself. Also not the email service) Then create an SMTP Username https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/smtp-authentication?tabs=built-in-role

Send an email:

-Server: smtp.azurecomm.net

-Port: Port 587 (recommended) or port 25

-TLS / StartTLS: Enabled

-Username: [The SMTP username you created in step 6]

-Password: [The Secret 'Value' you created in Step 5]

And your DONE! On a quick run this setup took 8 mins to do the whole setup.
Veselý, Ondřej 5 Reputation points

2024-05-28T12:08:35.93+00:00

I have exactly the same issue in UniFlow, Canon printers and in Navision(basically all my SMTP use cases).

I would assume that the length of the username AND password is going to be a major issue pretty much in majority of the applications where this could be used.
Mischa Rohleder 0 Reputation points

2024-05-29T13:36:28.4033333+00:00

I am having the same problem.. Is there any update?

We are cut at 64 characters
Thomas Oeser 0 Reputation points

2024-07-23T15:51:05.92+00:00

@brtrach-MSFT , any news regarding the character length issue for the username? We have some large enterprise customers willing to test the Azure Communication services relay functionality but this is a blocker for them. Especially printers have a character length limit of 64 characters for username field but as others already mentioned other applications have similar restrictions. This needs to be addressed to make this product usable.
Lorenzo Cernecca 6 Reputation points

2024-08-05T06:32:12.1566667+00:00

I have similar needs looking to use ACS as a SMTP solution for devices that don't support modern authentication. An example is a Dell iDRAC which appears to have a 50 character username limit.

If I only include the <Entra Application ID>|<Entra Tenant ID> component of the ACS username I am seeing 75 characters which makes it useless for this purpose. As I created a descriptive <Azure Communication Services Resource name> the combined total of my username came to 111 humorous characters.
Joe Widi 25 Reputation points

2024-09-05T20:36:03.94+00:00

I have this same issue; the username is entirely too long for even a brand-new app we're installing. We are actually stuck at a 50-character limit.
Brian House 0 Reputation points

2024-10-04T19:00:08.95+00:00

@brtrach-MSFT Did this user ever give you details to pursue this with the product group? I just ran into this exact issue today and will gladly relay my experience
Frank E 0 Reputation points

2024-10-07T18:14:32.9533333+00:00

@brtrach-MSFT I have the same issue. Can you please give us an update?
Frank E 0 Reputation points

2024-10-08T08:54:07.1833333+00:00

Is there any update on this? It's really annoying. Now i have to use a third party service.
Patrick Helm 5 Reputation points

2024-10-17T05:44:45.9466667+00:00

I'm facing a Kyocera vendor SMTP 64 character limit for username as well. :( All the effort to get the things working, to find this the blocker.
Alex Pettigrew 5 Reputation points

2024-11-04T22:50:08.59+00:00

If it eeks anyone below the limit their are facing in their application, you are able to remove the dashes in the app/tenant ID. But I am also looking for this to be fixed to be able to reliably use this product. Most legacy apps requiring smtp auth were simply not built with expectations of usernames this long.
Nebhuth, Garry 0 Reputation points

2024-11-20T15:39:56.49+00:00

next try..any updates ?
Link Martin 5 Reputation points

2024-12-11T14:14:10.6233333+00:00

@brtrach-MSFT Hi, is there any chance that the short usernames will come soon. We want to switch our MFP to this services, but we can´t because the usernames are to long.
Anne 10 Reputation points

2025-03-11T15:13:07.0366667+00:00

Does anyone know if Microsoft enabled the custom character limit for username?
de la Mar, Joeri 5 Reputation points

2025-03-12T12:22:35.9566667+00:00

Our organization also needs this, to be able to adopt this service properly. If this functionality will not be offered, ACS Email is a no-go.
Rick Carlson 10 Reputation points

2025-04-25T00:06:16.79+00:00

Has there been any movement from MS on allowing shorter usernames? I just went through the painstaking multi-day process of getting ACS working correctly, only to be unable to use it with 99.9% of our STMP clients. This is highly frustrating, as many of the services with this ~60 char username limit are from MS themselves!
Link Martin 5 Reputation points

2025-04-25T05:10:32.65+00:00

We did not recived any news about shorter Usernames. We have the same problem with the most of our MFPs and our ERP Systems. Only some are able to do oauth. The others normaly do not allow long usernames.

I cannot understand why MS is switching off the service in Exchange without providing another solutions whoch works.
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
David Wilson 0 Reputation points

2025-05-20T16:10:49.5466667+00:00

This is Nice! I was on support about this and they mentioned they were going to add the feature.

It does stink though that our xerox copiers don't take passwords larger than 31 characters. this makes it hard for me to use this smtp relay since a lot of what we use relay for is Xerox copiers. I think Microsoft's generated passwords are too large for this. if anyone has any workarounds I would love to know.
Emmitt Houston 0 Reputation points

2025-05-20T17:12:18.22+00:00

Depending on your licensing and need High Volume Email (HVE) may be an option for you. I have been working for a client that uses a Konica scan to email for mostly internal recipients.

Answer 1

Rasmus Hollin 20

Good news! I have found a new setting in Communication Services under Email: "SMTP Usernames". There you can add custom SMTP usernames, either as a email address, or as free text username. Then you change the long username consisting of <Azure Communication Services Resource name>|<Entra Application ID>|<Entra Tenant ID> to the custom username instead, and it just works!

User's image

When I first added a post in here, the status told me "Missing permissions" and details stating that the Entra Application did not have these permissions.
Microsoft.Communication/CommunicationServices/read
Microsoft.Communication/CommunicationServices/write

However, I found out that this is related to case sensitive comparision. The application had correct permissions, but Read and Write was with capital first letter. You can still use the username, but if you want to have a valid status, edit the custom role, open up the json editor and change to lowercase:

User's image

Nebhuth, Garry 0 Reputation points

2025-05-16T16:58:32.71+00:00

I can hardly believe it. It works. After creating an SMTP username, assigning permissions, and correcting the capital letter at the beginning, the service displays correctly and emails are sent. Thank you very much for this feedback.
Emmitt Houston 0 Reputation points

2025-05-20T07:58:59.14+00:00

@Rasmus Hollin Thank you for the post and screenshots detailing the JSON edit. Wow! great find on the case-sensitivity. I look forward to testing this again. I plan to leave the client I've setup with HVE as-is for not but, if the Azure Communication Service is going to work now that they've added the ability to add custom names, I'll transition to it going forward.
Ashley - Tetrabyte 25 Reputation points

2025-05-20T08:13:14.3633333+00:00
I can confirm this is a working solution.

For a full solution here is a quick fire guide on the steps:

Create an Azure Communication Service
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/create-communication-resource?tabs=windows&pivots=platform-azp

Create an Azure Communication Email Service (yes both are needed)
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/create-email-communication-resource?pivots=platform-azp

Add your Custom Domain to the Email Service
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/add-custom-verified-domains?pivots=platform-azp

Link that to the Communication Service
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/connect-email-communication-resource?pivots=azure-portal

Create and Register a App Registration
Create a Secret and save the value
https://learn.microsoft.com/en-us/entra/identity-platform/howto-create-service-principal-portal#option-3-create-a-new-client-secret

Add the Application to the Azure Communication Service (Not the Resource Group or Subscription, must be direct! on the Communication Service itself. Also not the email service)
Then create an SMTP Username
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/smtp-authentication?tabs=built-in-role

Send an email:
-Server: smtp.azurecomm.net
-Port: Port 587 (recommended) or port 25
-TLS / StartTLS: Enabled
-Username: [The SMTP username you created in step 6]
-Password: [The Secret 'Value' you created in Step 5]

And your DONE!
On a quick run this setup took 8 mins to do the whole setup.
James Epp 31 Reputation points

2025-06-11T18:13:36.6666667+00:00

I had no idea this was a thing. Does anyone know when Microsoft first released this feature? When I was first setting up ACS months ago this was definitely not there, or I'm going blind.
Rasmus Hollin 20 Reputation points

2025-06-12T05:17:08.7633333+00:00

@James Epp , no this was not in there from the beginning. My guess is that this what added during May. Wayback machine has a version of the documentation page from May 4th where this feature is not documented, and I found the feature in the middle of May.

Answer 2

brtrach-MSFT 17,731 Microsoft Employee Moderator

@Jonny Sharp Good news! The product group wants me to gather some information from you so they can look into whether or not it would be possible to address this limitation.

Can you please send me an email to [email protected] with the subject as ATTN: Bryan. I will then reply with the information we need for the product group to begin their consideration.

Peter Paul Bartolo 5 Reputation points

2024-01-03T08:55:07.26+00:00

Was there any uptake/progress on this?
Jonny Sharp 110 Reputation points

2024-01-04T15:14:13.5766667+00:00

Hi @brtrach-MSFT . Sorry for not responding sooner, I didn't get a notification about your messages. I've just send the email to you as requested.
Kind regards,
Jonny
Jonny Sharp 110 Reputation points

2024-01-04T15:24:22.01+00:00

Hi @brtrach-MSFT - further to my message above, my email to [email protected] is being rejected.
brtrach-MSFT 17,731 Reputation points Microsoft Employee Moderator

2024-01-09T17:07:15.44+00:00

@Jonny Sharp My apologies but I was out of the office for the holidays and just came back today.

Mia reached out to me and let me know you found a way to make contact via support case.

The product group is hoping to address this matter in a future update. Mia will work with you on gathering your use case scenario to ensure the product group can address your issue in the coming months.

Let me know if I can be of further assistance.
Logan Cook 5 Reputation points

2024-01-25T21:16:31.8966667+00:00

We're actively looking for a solution to this as well. Other than this limitation, the preview has been working beautifully.
Nyberg Mika 0 Reputation points

2024-06-26T12:11:29.96+00:00

Is there a solution for this? I am facing the same issue with Microsoft Dynamics365 FO smtp settings, 64 being maximum length for username
Joe Widi 25 Reputation points

2024-09-05T20:43:00.6433333+00:00

@brtrach-MSFT Any updates on this particular issue?
Logan Cook 5 Reputation points

2024-10-04T17:56:50.4866667+00:00

Entra application IDs are unique as-is. Including the tenant ID in the username is entirely redundant. It'd also be possible to have it accept either the version with or without tenant ID. There's no good reason for this still being an issue after almost a year.
Frank E 0 Reputation points

2024-10-08T08:53:12.32+00:00

@brtrach-MSFT Is there any update on this?
Molenbuur, Imko 0 Reputation points

2024-12-16T21:49:59.91+00:00

@brtrach-MSFT can you please give an update on this issue, we're facing the same issue for one of our customers on a SAP S/4HANA 2023 system.
Emmitt Houston 0 Reputation points

2024-12-25T05:51:18.7433333+00:00

I'm setting this up as well on an HP MFP M283fdw (table top MFD) that does not support OAuth2.0. The username and password fields are limited so the password field won't fit the client secret and the username fit but, will likely be silently truncated. This is also going to be an issue on a Konica Minolta Bizhub C258i also using scan to email smtp with no OAuth2.0 support.

Should I follow the same guidance from 2023 and email [email protected] with the subject as ATTN: Bryan?
Thackeray, Sam 0 Reputation points

2025-02-10T13:59:04.6866667+00:00

Hi,
Adding my +1. I have also run into this issue with our Canon MFP's only supporting usernames up to 64 characters.
Niek Strijker 0 Reputation points

2025-02-19T12:04:54.6333333+00:00

Is there any update on this as I am running into the same issues with the long username
David Wilson 0 Reputation points

2025-04-14T16:43:31.77+00:00

@brtrach-MSFT Any update to this? We are looking into switching to this for our organization, but cannot because our xerox copiers have a 64 char limit

Answer 3

Hey Jonny,

Ah, the joys of preview features—where the excitement of cutting-edge tech meets the frustration of "why won’t this just work?!" Your 94-character username is like trying to fit a novel into a tweet some clients just can’t handle the drama.

Sadly, the Service Principal format is non-negotiable (thanks, Azure), and UPNs are a no-go (error 535 says "nice try, though"). For now, you’re stuck with clients that can handle your epic username saga. Maybe it’s time to start a support group for SMTP clients with character limits?

As a workaround, you could try using a proper SMTP relay (I know, I know, it’s like going back to dial-up after fibre) or beg Microsoft to shorten the username in a future update. Fingers crossed they’ll fix it before the feature goes GA!

Keep fighting the good fight, and let me know if you crack it.
Cheers! Alex

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This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.

Answer 4

Emmitt Houston 0

I was hoping this would work better as well. I went with the High Volume Email (HVE) Account for my client. I tested it internally and it works, I’ll be implementing it with my client this week. Additionally, it doesn’t require a license. The Azure Communication Services probably works best for apps vs devices. The HVE Account has external limitations but, they should be more than sufficient for devices scanning to email.

Emmitt Houston 0 Reputation points

2025-02-19T13:33:26.69+00:00

https://techcommunity.microsoft.com/blog/exchange/exchange-online-to-retire-basic-auth-for-client-submission-smtp-auth/4114750

https://learn.microsoft.com/en-us/Exchange/mail-flow-best-practices/high-volume-mails-m365

Answer 5

James Epp 31

ACS is not RFC compliant.

https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.1.1

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Username length issue with SMTP in Azure Communication Services

5 answers

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