Microsoft 365 5.5.0

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.

In practice

550 5.5.0 Requested action not taken: mailbox unavailable is what Outlook.com and Hotmail return for an address that does not exist. Microsoft says it is similar to 550 5.1.10. It is a hard bounce. Outlook.com addresses are also among the hardest to verify by SMTP, because Outlook.com often accepts every address during the conversation, so some unknown addresses are only found at send time. Check for typos in the local part before removing the address. If you see many of these from one campaign, the list has a high share of dead Outlook.com addresses and should be verified before the next send.

What the sender should do

Treat it as a hard bounce: check the address for a typo, then remove it from the list.

Common questions

What does Microsoft 365 5.5.0 mean?

Microsoft 365 5.5.0 means: 550 5.5.0 Requested action not taken: mailbox unavailable. 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 5.5.0?

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 5.5.0?

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.