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Azure Communications Services - Email - MailboxNameNotAllowed - subdomain

Mihai Mihaiescu 60 Reputation points
2025-05-27T10:16:54.1566667+00:00

Hello,

I have an azure communication services domain configured correctly everything is validated.

I keep getting my emails failed with the error "MailboxNameNotAllowed"

The anonimized details:

#5.1.8 Domain of sender address ******@subdomain.domain.com does not exist};{MSG=};{FQDN=xxxx.dm.de};{IP=x.x.x.x};{LRT=5/27/2025 9:32:31 AM}]'

Azure Communication Services
Azure Communication Services
An Azure communication platform for deploying applications across devices and platforms.
1,210 questions
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  1. Grmacjon-MSFT 19,151 Reputation points Moderator
    2025-06-06T15:58:01.68+00:00

    Hi @Mihai Mihaiescu I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this!

    Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.

    Issue :

    Your ACS domain configured correctly everything and everythingwas validated but you kept getting your emails failed with the error "MailboxNameNotAllowed"

    Solution :

    After investigating this issue you confirmed that it was Azure related. You discovered that some enterprise servers reject your emails if the subdomain from which the email comes even though it is valid in ACS. So in order to send emails from a custom domain, you don't have to pass only the ACS validation rules and Azure recommendations, but you also have to do this A setting for the subdomain, to a valid IP.

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  1. Mihai Mihaiescu 60 Reputation points
    2025-05-29T17:45:38.07+00:00

    Hello, thank you both for the support,

    I solved it but it was totally different issue, some enterprise servers reject your emails if the subdomain from which the email comes (even though is valid on Azure CS) does not have a valid "lookup"-able A value. This is so obscure you would never think of it, but unfortunately some enterprises apply this rule.

    So to be ok to send emails from a custom domain, you don't have to pass only the Azure Communication Services validation rules and apply the recommendations made by Azure, you also have to do this A setting for the subdomain, to a valid IP. Even though it does not make sense, it is what it is...

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