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550 5.1.1 error: the address doesn't exist

550 5.1.1 means the recipient's mailbox doesn't exist: a typo, a deleted account or a guessed address. It's a list problem. Remove the address and move on.

List problemThe address is the problem. Clean the list; your domain is fine.

What 550 5.1.1 means

Any server5.1.1 Bad destination mailbox address

The standard meaning: the mailbox named in the address doesn't exist at that domain.

Gmail550 5.1.1 The email account that you tried to reach does not exist.

Gmail's wording. A typo, an account that's been deleted, or an address that was never real.

Microsoft 365 / Outlook550 5.1.1 Bad destination mailbox address

Microsoft notes it also shows up when an address that used to work was changed or removed.

List problem or sender problem?

List problem. The receiving server looked the address up and found no such mailbox. Nothing about your domain was judged. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 550 5.1.1

  1. Check the address for typos and extra spaces.
  2. Remove it and don't retry. Yahoo's guidance for permanent errors is exactly that: take the address off your list.
  3. If it used to work, the person likely left or the mailbox was renamed. Find their new address another way.
  4. If many 5.1.1s come from one list or source, stop sending to that list and check it before the next step goes out.

In SendingOS, a 550 5.1.1 bounce shows its reason and code on the conversation, and the thread is labelled Bounced, so you can filter the inbox to bounces and pull these addresses before they add up.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

550 5.1.10550 5.4.1553 5.1.3550 5.2.1

Sources: Google: Gmail SMTP errors and codes · Microsoft: NDRs in Exchange Online · Yahoo: SMTP error codes · RFC 3463

Questions

Can 550 5.1.1 bounces hurt my domain?

One won't. Many will: mailbox providers read a lot of mail to addresses that don't exist as a sign of a careless sender. Remove these addresses as soon as they bounce.

Should I retry a 550 5.1.1?

No. It's permanent: the mailbox isn't there. Retrying only adds another bounce.