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SMTP 551: user not local, try another path

551User not local; please try forward-pathPermanent: retrying will not help

Typical wording
551 5.1.6 User not local; please try <user@example.net>

What it means

RFC 5321 defines 551 as "User not local; please try <forward-path>". The server knows the recipient but will not accept mail for them here, and may give the address where they now receive mail. The sending side is expected to update its records and send to the new address, not have the server forward it.

Its friendly twin is 251, "User not local; will forward", where the server accepts the message and passes it on itself. RFC 5321 notes that both are optional and that servers may prefer to return 550 instead, to avoid revealing information about users.

In practice 551 is uncommon. When it appears it usually pairs with 5.1.6, "Destination mailbox has moved, no forwarding address", or with relay wording on servers that misuse it to refuse mail for domains they do not host.

Likely causes

  • The recipient moved to a new address or domain and the old server tells senders to use the new one.
  • A university or company retired an alias and points senders at the current address.
  • Some older servers return 551 to refuse relaying for domains they do not handle.
  • An MX record for the recipient domain points at a server that no longer hosts that domain.

How to fix it

  1. Read the reply for a suggested address. If one is given, confirm it with the recipient and update your contact or list entry.
  2. Do not resend to the old address; 551 is permanent and the server will refuse it again.
  3. If no address is offered, contact the recipient another way and ask for their current email.
  4. If you own the recipient domain and see 551 in logs, check that every MX record points at a server that still hosts the domain.

Checks that apply

  • MX Lookup Shows which servers handle the recipient domain, revealing a stale MX that points at a host no longer serving it.
  • SMTP Diagnostics Connects to the listed MX so you can confirm it is the mail system the domain owner expects.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Should my list software unsubscribe a 551 address?
Yes, treat it as a hard bounce for that address. If a new address is given, only use it once the person confirms they want mail there.
Why not just forward the message?
The receiving server chose not to forward; 551 hands that decision back to the sender instead.

All 191 bounce codes