Microsoft 365 5.1.1
Bad destination mailbox address
Microsoft 365 5.1.1 means: bad destination mailbox address. It is a permanent failure, so do not retry it unchanged. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Bad destination mailbox address
Possible cause
The following conditions might cause this failure: The sender incorrectly entered the recipient's email address. The recipient's email address doesn't exist in the destination email system. The recipient's mailbox was moved, and the sender's Outlook recipient cache didn't update. An invalid legacy domain name (DN) exists for the recipient's mailbox Active Directory Domain Service.
Provider's advice
This error typically occurs if the sender of the message enters an incorrect email address of the recipient. The sender should check the recipient's email address and send again. This error can occur if the recipient's email address was previously valid but later changed or removed. If the sender of the message is in the same organization as the recipient, and the recipient's mailbox still exists, determine whether the recipient's mailbox was relocated to a new email server. If so, Outlook might not update the recipient cache correctly. Instruct the sender to remove the recipient's address from sender's Outlook recipient cache, and then create a new message. Resending the original message causes the same failure.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.1.1: Bad destination mailbox address |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | Yes, verification catches this |
In practice
Microsoft lists several causes: a mistyped recipient, an address that does not exist in the destination system, a mailbox moved while the sender's Outlook cache still holds the old entry, and an invalid legacy DN in Active Directory. For outside senders the first two are what matters, and the address should be removed. Inside an organisation, clearing the Outlook autocomplete entry often fixes it.
What the sender should do
Treat it as a hard bounce: check the address for a typo, then remove it from the list.
Would verifying the list have prevented Microsoft 365 5.1.1?
Yes, verification catches this
A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Common questions
- What does Microsoft 365 5.1.1 mean?
- Microsoft 365 5.1.1 means: bad destination mailbox address. It is a permanent failure, so do not retry it unchanged. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
- Should I retry after Microsoft 365 5.1.1?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
- Would verifying my list have prevented Microsoft 365 5.1.1?
- Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
The same code elsewhere
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05