Outlook.com calendar invitations are being blocked with error 550 5.7.520

Question

Outlook.com calendar invitations are being blocked with error 550 5.7.520

Mauro Carvalho 0

Regular emails are delivered successfully.

Issue only affects calendar invites (ICS / Teams).

When recipients accept the invitation, a delivery failure occurs with:

550 5.7.520 – Message blocked because it contains content identified as spam

550 5.5.0 – mailbox unavailable (S2017062302)

Additionally, responses are routed to an internal Outlook address:

******@outlook.com

This strongly indicates an internal Outlook.com calendar routing or filtering issue.

Has anyone experienced this or can confirm if this is a known issue?

EmilyS726 227K Reputation points Independent Advisor

2026-04-27T21:14:28.79+00:00

Hello

Do you send your Teams invitation via Outlook Calendar, or via Teams calendar?

Any chance you are using a non-Microsoft domain email address as your Microsoft account username, meaning, your username is NOT outlook.com or hotmail.com?
Mauro Carvalho 0 Reputation points

2026-04-28T15:13:29.1533333+00:00
Hello @EmilyS726

Thanks for your reply.

The issue has been reproduced in multiple scenarios:

Invitations sent via Microsoft Teams

Invitations sent via Outlook Calendar

The behavior is the same in both cases.

Regarding the account:

The sender account used for testing is a standard Outlook.com account (******@outlook.pt)

No external domain is used as the Microsoft account username

Additionally:

Regular emails are delivered without any issues

The problem only affects calendar invitations (ICS)

The failure occurs when recipients accept the invitation

The response is routed to an internal Outlook address (******@outlook.com), which fails

This suggests an issue in Outlook.com calendar processing or internal routing rather than account configuration.

Let me know if you need more technical details or headers/logs.

2 answers

Your answer

EmilyS726 227K Reputation points Independent Advisor

2026-04-27T21:14:28.79+00:00

Hello

Do you send your Teams invitation via Outlook Calendar, or via Teams calendar?

Any chance you are using a non-Microsoft domain email address as your Microsoft account username, meaning, your username is NOT outlook.com or hotmail.com?
Mauro Carvalho 0 Reputation points

2026-04-28T15:13:29.1533333+00:00

Hello @EmilyS726

Thanks for your reply.

The issue has been reproduced in multiple scenarios:

Invitations sent via Microsoft Teams

Invitations sent via Outlook Calendar

The behavior is the same in both cases.

Regarding the account:

The sender account used for testing is a standard Outlook.com account (******@outlook.pt)

No external domain is used as the Microsoft account username

Additionally:

Regular emails are delivered without any issues

The problem only affects calendar invitations (ICS)

The failure occurs when recipients accept the invitation

The response is routed to an internal Outlook address (******@outlook.com), which fails

This suggests an issue in Outlook.com calendar processing or internal routing rather than account configuration.

Let me know if you need more technical details or headers/logs.

Answer 1

Ok, I think I know what you mean now. Thank you for the additional details, super helpful.

The problem is due to sender's primary alias being a legacy outlook email address (outlook.pt).

Bottom line is - this will not affect the Teams meeting on both sides. Teams meeting should still be saved on both users' calendar. So there's nothing to worry there.

The cause of the problem is because calendar and Teams calendar need to use a standard outlook.com address to send the invite when your account's alias is either a third party email address, or a legacy email address (like the one with the locale code vs. the .com). This email address is for sending purpose only. It is one with random number and letter. It will send, but it cannot receive. This is why when they accept the meeting and send the confirmation, it will hit the error "unknow mailbox".

You can ignore this issue as it will not affect any functionality, well, other than you not getting the confirmation emails.

Or you will need to give your account a proper outlook.com alias by using https://account.live.com > Your info > sign in preference > create new.

Answer 2

This behavior matches a known Outlook.com issue where messages with certain attachments are incorrectly flagged as spam with error 550 5.7.520.

In Outlook.com, some users are currently unable to send emails that contain attachments and receive:

550 5.7.520 Message blocked

Calendar invitations (ICS / Teams) are also messages with attachments and can be affected by the same filtering problem, even when regular mail works. In that case, the invite or the response can be blocked by Outlook.com’s spam/content filters, resulting in 550 5.7.520 and related delivery failures.

Status and current guidance:

The issue is acknowledged and under fix.
While it is being fixed, the recommended workaround for affected messages with attachments is to send the content via OneDrive link instead of as an attached file.

For calendar scenarios, this means:

Where possible, avoid embedding large or complex attachments directly in the invite.
Share files via OneDrive links in the body of the invite rather than as attachments.

If the problem persists for simple, attachment‑free invites, it is likely still the same underlying filtering issue and will require waiting for the service-side fix or opening a support case via Outlook.com help.

References:

Fixes or workarounds for recent issues on Outlook.com

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Outlook.com calendar invitations are being blocked with error 550 5.7.520

2 answers

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