Outlook is trying to send new emails and meeting invites via Teams

Question

I'm getting error messages occassionally saying "XXX is not authorized to relay messages through the Office 365 email servers for teams.mail.microsoft."

Except I don't have any Teams email addresses saved anywhere.

This has happened a couple of times. Once with an external email that I simply replied to. Then yesterday when I created a meeting invite for a team member and I didn't notice that it loaded up an email address with the person's name@ ******@teams.mail.microsoft instead of his work email. I checked my saved contacts and auto-fill options and neither of them have this name saved in them. Any idea why this might be happening?

Answer 1

The behavior is caused by how Outlook and Microsoft 365 resolve and route addresses, not by a visible contact that was manually saved.

Key points based on the symptoms:

1. The name@…@teams.mail.microsoft address is an internal routing-style address
   • Microsoft 365 and Teams can use special proxy or routing addresses (for example, service-style addresses) that are not shown in normal contact lists.
   • When Outlook resolves a name, it can pick up one of these hidden or proxy-style addresses instead of the user’s primary SMTP address, which then causes relay or authorization errors when used as the sender/recipient on the open internet.
2. Outlook can resolve to unexpected directory attributes
   • Outlook name resolution does not only use the visible SMTP address; it can also match on other directory attributes.
   • As documented for other mis‑routing issues, Outlook can resolve a character or string to a user based on attributes such as Office, telephone, home, or mobile, and then silently substitute that value as the recipient. This is why messages can end up addressed to an unexpected internal-style address even though it was not explicitly chosen from contacts or autocomplete.
   • The same underlying resolution mechanism can cause Outlook to pick a non-primary address (such as a Teams- or service-related proxy address) for a user instead of the normal work email.
3. Why the relay error appears
   • Office 365 SMTP servers enforce anti‑spoofing and relay restrictions. If Outlook tries to send from or to a teams.mail.microsoft-style address that is not allowed to relay externally, the server returns a 550/relay‑prohibited style error.
   • This is consistent with the documented behavior where servers block messages that appear to be spoofed or that use an address/domain not permitted for that connection.
4. What to check and how to reduce recurrence
   • When composing or replying, always verify the resolved address by hovering over or double‑clicking the recipient to ensure it is the user’s correct primary SMTP address, not a …@teams.mail.microsoft variant.
   • If a specific incorrect address keeps appearing, remove it from any nickname/AutoComplete cache entry in Outlook (by deleting it from the suggestion list when it appears in the To field) so Outlook is less likely to reuse it.
   • If the issue persists for a particular user, an administrator can review that user’s directory/proxy addresses in Microsoft 365 to ensure there is no conflicting or misleading proxy address that Outlook might be resolving to.

These steps address the underlying name-resolution and relay behavior that leads to Outlook occasionally using a teams.mail.microsoft address and triggering the “not authorized to relay” error.

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References:

• I receive a 550, 553, or relay-prohibited error when sending email messages
• Unintentional emails received in Outlook