Undeliverable emails

Question

my outgoing email messages to gmail accounts keep coming back undeliverable. How do I fix?

Answer 1

Could you please post a screenshot of the full undeliverable message/NDR with personal information hidden, especially the part after the error code? Also, please confirm whether you are sending from Outlook.com web only, or from a mail app connected to Outlook.com/another custom domain.

Answer 2

Undeliverable messages usually mean Outlook.com or the receiving server rejected the message. Use the non-delivery report (NDR) that comes back to identify the cause and fix it.

1. Open one of the bounced messages.
2. Scroll down and look for a section similar to “Diagnostic information for administrators” or an SMTP error like 550 or 553.
3. Based on what it says:
   • If the Gmail address is wrong or misspelled, correct the address and resend.
   • If the report says the recipient doesn’t exist, confirm the Gmail address with the contact.
   • If it mentions message size or attachments (for example, larger than 10 MB), remove or compress large attachments and resend.
   • If you see a 550, 553, or “relay prohibited/relaying denied” type error, the server is rejecting the message as not authorized to send. In that case, send from Outlook.com/Web (not an old POP/SMTP profile), or verify the account is properly authenticated in the mail app being used.
4. Make sure the bounce is a legitimate message from ******@outlook.com, postmaster@microsoft.com, or service.microsoft.com, and not a spoof. If it looks suspicious, follow the guidance in Phishing and suspicious behavior in Outlook.
5. After correcting the specific issue indicated in the diagnostic section, resend the email.

If the NDR does not clearly match any of the above, repeat step 2 and focus on the exact SMTP error text; that text is what determines the next action.

---

References:

• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com
• I receive a 550, 553, or relay-prohibited error when sending email messages