Microsoft mail servers 550 5.1.10
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.
Decision
Suppress this recipient. A retry is unlikely to help.
Confidence: high. Recipient validity: invalid.
Match this reply
- SMTP stage
- RCPT TO recipient check
- Endpoint
- Inbound MX
Recipient not foundUse 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
- Classify as a hard bounce only when this provider-specific fingerprint matches.
- Stop retries for the bounced recipient and keep the raw SMTP reply for audit and support.
- Do not suppress other recipients in the same send unless they return their own matching bounce.
Avoid
- Do not suppress from the numeric code alone. Confirm the provider, SMTP stage, and diagnostic text first.
- Do not keep retrying once the provider-specific evidence says the mailbox is invalid.
- Do not apply this suppression decision to the same code from another provider without matching that provider's 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