EC
251
SMTPINFOCriticalSuccessful DeliveryHIGH confidence

User not local; will forward to <forward-path>

What this means

The 251 code indicates that the recipient is not on the local server, but the server will accept the message and forward it to the correct destination. The sending client's responsibility for delivery ends here.

Why it happens
  1. 1The recipient address has a forwarding rule set up on the server.
  2. 2The server is acting as a mail gateway or relay for another mail system.
How to reproduce

A client sends an email to an address that automatically forwards to another external email account.

trigger — this will error
trigger — this will error
RCPT TO:<old-address@example.com>
251 2.1.5 User not local; will forward to <new-address@another-domain.com>

expected output

251 User not local; will forward

Fix

Treat 251 as successful acceptance and continue the SMTP transaction

WHEN When RCPT TO returns 251 instead of 250

Treat 251 as successful acceptance and continue the SMTP transaction
RCPT TO:<forwarded@example.com>
251 User not local; will forward to <real@anotherdomain.com>
# Continue with DATA normally

Why this works

251 is a positive 2xx completion reply. The server has accepted responsibility for forwarding. The sending client's job is done; continue with DATA.

What not to do

✕ Treat 251 as an error or abort the transaction

251 is a success code. Aborting wastes a server-accepted delivery slot and prevents the message from being sent.

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