5.1.0 Microsoft bounce: sender denied

What 5.1.0 means

Exchange Online could not resolve the recipient or sender address for this message, so it returned a sender-denied NDR.

Message text

Sender denied

Common causes of 5.1.0

• Message was saved to a file, opened offline and replied to, so the recipient lookup can fail
• Recipient address is incorrectly formatted

How to fix 5.1.0

1. Check the recipient address and send the message again.
2. Pick the recipient from the address book rather than an old saved copy of the message.

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 5.1.0

The second part, 5.1.0, 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 1 is addressing: a problem with the recipient or sender address. 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 set up MX records (priorities, null MX, migration)
• MX host does not resolve: causes and how to fix it
• Null MX (RFC 7505): publish it for domains with no mail

Related codes

• 5.1.1 SMTP: bad destination mailbox
• 5.1.2 SMTP: bad destination system
• 5.1.3 SMTP: bad address syntax
• 5.1.10 SMTP: recipient has null MX
• 550 5.1.1 Microsoft: bad mailbox address
• 550 5.1.10 Microsoft: recipient not found

FAQ

All bounce codes