Permanent failure — do not retry
SMTP 550
mailbox unavailable
SMTP 550 means the requested action was not taken because the mailbox is unavailable. RFC 5321 gives "mailbox not found", "no access" and "command rejected for policy reasons" as examples. It is permanent. The enhanced code after it tells you which: 5.1.1 is an unknown user, 5.7.x is a policy block.
How it appears
550 Requested action not taken: mailbox unavailable
550 5.1.1 User unknown
550 5.7.1 Message rejected due to local policy
What causes it
- The mailbox does not exist (550 5.1.1), the most common case.
- The mailbox is disabled or suspended (550 5.2.1).
- The message was refused for policy reasons, such as failed authentication or a blocklisted IP (550 5.7.x).
- The server will not relay for the domain you are sending to.
How to fix it
In order — try the first one first.
- Read the enhanced code. If it starts 5.1, remove the address. If it starts 5.7, fix your sending setup instead.
- Do not retry a 550 for the same address. Repeated sends to dead mailboxes damage sender reputation.
- If a large share of a send returns 550 5.1.x, the list is stale. Verify the rest before the next send.
Common questions
- What does SMTP 550 mean?
- SMTP 550 means the requested action was not taken because the mailbox is unavailable. RFC 5321 gives "mailbox not found", "no access" and "command rejected for policy reasons" as examples. It is permanent. The enhanced code after it tells you which: 5.1.1 is an unknown user, 5.7.x is a policy block.
- Should I retry a 550 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550?
- Read the enhanced code. If it starts 5.1, remove the address. If it starts 5.7, fix your sending setup instead.
Specific 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11