SMTP code 550 is a hard bounce: the receiving server has refused the message. The text right after the code — « mailbox unavailable », « user unknown », « message rejected », « blocked by policy » — is what tells you why. Treat 550 as: stop sending to this address until you understand the reason.
Quick rule 550 + « user unknown / no such user » = the address does not exist. Remove it. 550 + « blocked » or « policy » = the server accepts it but rejects you, so check your authentication and reputation.
The most common 550 sub-reasons
| Reason in the bounce | What to do |
|---|---|
| User unknown / no such user | Permanent. Remove the address from your list. |
| Mailbox full | Soft case — full mailbox bounce. Wait, then retry once or twice. |
| Blocked / blacklisted IP | Check Mailpro’s sending IP reputation; pause the campaign. |
| Authentication required / SPF or DKIM fails | Re-check SPF, DKIM and DMARC on your sending domain. |
| Message content rejected | Lighten the content, remove suspicious links or attachments. |
What Mailpro does for you
Hard 550 bounces (user unknown) are tracked in your bounces list and excluded from future sends — see bounces management. Soft cases (mailbox full) are retried automatically before being escalated.
Don’t resend blindly Retrying the same address after a 550 user-unknown is the fastest way to damage your sender reputation. Always remove the address first.
Understand and fix your bounces
Read everything you need to know about bounces, the difference between hard bounce and soft bounce, and why managing bounces matters.