What does 551 mean?

You sent an email and got back a 551. The receiving server is telling you one thing clearly: the address you targeted isn't handled here. It might have moved to a different server, or you might simply have the wrong address.

A 551 is a permanent 5xx failure, so it's treated as a hard bounce. Don't keep retrying the same address. That only damages your sender reputation.

The message you get back usually looks something like one of these:

• "551 User not local"
• "551 Please try [forwarding address]"
• "551 Not our customer"

If the server includes a forwarding address in that message, it's telling you where to try instead. That said, it's worth confirming with the contact directly before updating your records, since forwarding paths can be outdated too.

What to check when you see a 551

• Did you spell the address correctly? Typos are the most common cause. Double-check the domain and the local part (the bit before the @).
• Has the contact moved? If this is someone you've emailed before and it suddenly fails, their mailbox may have migrated to a new server or domain.
• Is this a domain routing issue? Some servers pass mail to others for delivery. A 551 can appear if the routing configuration on the receiving end is broken, not because the address is wrong.
• Did you get a forwarding address in the error? Use it, but verify it's current before adding it to your list.

From a list hygiene standpoint, a confirmed 551 means you should suppress that address immediately. Continuing to send to addresses that bounce hard is one of the faster ways to hurt your domain reputation with receiving servers.

If you're seeing 551 errors across a batch of contacts, that's a signal your list may need a proper clean. You can check what's recoverable with RME Clean, or if something feels off with your sending setup, drop into our SOS hotline and we'll take a look.