5.7.134 Microsoft bounce: mailbox blocks outside mail
What 5.7.134 means
The recipient is a mailbox set up to reject messages from outside its organisation, so your message was refused.
Message text
Sender was not authenticated for mailbox
Common causes of 5.7.134
- Mailbox is configured to accept only internal senders
How to fix 5.7.134
- Only the recipient organisation's email administrator can change this; ask them to allow the sender.
- Contact the recipient by another channel and ask them to allow your address.
Check your domain
Read the headers of a bounced messageRuns in your browser; nothing is uploaded.
Get your domain's deliverability score
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 5.7.134
The second part, 5.7.134, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 5.7.0 SMTP: security status
- 5.7.1 SMTP: delivery not authorized
- 5.7.23 SMTP: SPF validation failed
- 5.7.25 SMTP: reverse DNS failed
- 5.7.26 SMTP: authentication failed
- 5.7.27 SMTP: sender has null MX
FAQ
What does 5.7.134 mean?
The recipient is a mailbox set up to reject messages from outside its organisation, so your message was refused. This is a permanent failure.
How do I fix 5.7.134?
Only the recipient organisation's email administrator can change this; ask them to allow the sender. Contact the recipient by another channel and ask them to allow your address.
Is 5.7.134 permanent, and should I retry?
5.7.134 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.