550 5.1.1 — Recipient address does not exist
The recipient address is the problem
SMTP error 550 5.1.1 is a genuine hard bounce: the receiving server confirms no mailbox exists at that address. This is one of the few errors that really is about the recipient, and the address should be permanently suppressed so it is never sent to again.
What the server actually says
550 5.1.1 The email account that you tried to reach does not exist. Please try
double-checking the recipient’s email address for typos.
550 5.1.1 https://support.google.com/mail/?p=NoSuchUser - gsmtpWhat you see
A specific address fails every time while the rest of the list delivers normally. Common immediately after importing an old or purchased list.
Why it happens
- The address never existed - a typo, or a guessed pattern that does not match the organisation.
- The employee left and the mailbox was deleted rather than redirected.
- The domain exists but has no catch-all and the local part is wrong.
- The list was bought or scraped, so a large share of addresses are invented.
How to fix it
- Suppress the address permanently. This is the one case where suppression is correct. Keep sending to a confirmed-dead address and your bounce rate - a primary blocklist trigger - climbs for no return.
- Verify the whole list if several appear at once. A cluster of 5.1.1s means the list is bad, not that one address is. Run verification before sending the remainder.
- Keep bounce rate at or below 1-2%. Above that, providers read list-buying behaviour and start filtering the domain regardless of content.
- Stop sending while bounce rate is elevated. Pause, clean, then resume. Continuing to send through a high bounce rate is what converts a list problem into a reputation problem.
What not to do
- Do not retry the address. 5.1.1 is authoritative and permanent - the server is telling you the mailbox does not exist.
- Do not blame your own configuration. Unlike most 5.x errors, this one genuinely is the recipient, and your SPF/DKIM/reputation are not involved.
- Do not assume the domain is bad. Other addresses on the same domain are usually perfectly valid.
Frequently asked questions
Is 550 5.1.1 my fault?
Only in the sense that the address came from your list. The error itself is purely about the recipient not existing, and no change to your authentication or reputation will make it deliver.
What bounce rate is safe?
At or below 1-2%. Beyond that, providers treat the pattern as evidence of a purchased or stale list and begin filtering the sending domain.
Is 550 5.1.1 the same as 550 5.7.1?
No, and the difference matters a lot. 5.1.1 means the address is dead - suppress it. 5.7.1 is a policy refusal about you - fix authentication or reputation and the same address will accept mail.
How MailPilot helps
MailPilot classifies this error automatically on every connected mailbox and tells you which side is at fault, so a throttle is never mistaken for a dead recipient and a credential failure never suppresses a healthy address. It also monitors real inbox placement and checks SPF, DKIM and DMARC on every mailbox, which is where most policy rejections start.
Related errors
Last reviewed 2026-10-11 by the MailPilot team.