Glossary
SMTP 550
A permanent rejection meaning the mailbox is unavailable — typically it does not exist, access was denied, or the command was rejected for policy reasons. This is the classic hard bounce.
In plain terms
That address is not going to accept your mail. Ever, in its current form.
Why it matters for deliverability
550 is the single most important code in list hygiene. A high 550 rate tells mailbox providers you are mailing addresses you have not verified, and it is one of the fastest ways to damage sender reputation.
What to do about it
- Check the enhanced status code first. 5.1.1 (no such mailbox) is a list-quality problem and the address should be suppressed; 5.7.x is a policy or authentication rejection about you, and suppressing the recipient would remove a valid subscriber while leaving the cause in place.
- Suppress permanently only once you have confirmed the rejection is about the mailbox.
- If 550s spike, verify the list before the next send rather than after.
Source
RFC 5321 — Simple Mail Transfer Protocol
“550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)”
Checked 2026-08-29
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Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
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