Microsoft mail servers 550 5.5.0
Recipient issue
The provider is rejecting the mailbox or account. Only treat it as invalid when the provider-specific text, endpoint, and stage all point to recipient validation.
Use diagnostic text before suppressing.
Decision
Review the full SMTP reply before retrying.
Confidence: medium. Recipient validity: probably invalid.
Match this reply
- SMTP stage
- RCPT TO recipient check
- Endpoint
- Inbound MX
mailbox unavailableRequested action not takenUse tools
Start here
- 1
Confirm the recipient address and domain are spelled correctly.
- 2
Suppress the recipient only when the provider-specific evidence says the mailbox is invalid.
Confirm the cause
- Validate the recipient address and domain before suppressing it.
- Check whether the mailbox, alias, group, or contact exists and can receive external mail.
- Keep the full SMTP reply with the contact record so support can distinguish invalid addresses from policy blocks.
App handling
- Route to manual review or a conservative fallback classification.
- Do not suppress the recipient from this rule unless a later bounce gives stronger recipient evidence.
- Limit automated retries until the provider-specific cause is understood.
Avoid
- Do not decide from the SMTP code alone.
- Do not suppress unless the provider-specific evidence points to an invalid recipient.
- Do not retry high-volume traffic before you understand the diagnostic text.
Common causes
- The address is misspelled, deleted, disabled, or not mail-enabled.
- The recipient domain accepts mail, but the specific mailbox cannot receive this message.
- An alias, distribution group, or forwarded address no longer maps to a valid mailbox.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampenvelope recipientcontact id