Error '550 Unknown user' resetting connection

Question

• In SMTP connection, after passing wrong email address to RCPT command, we are receiving "550 Unknown user" error from one email server and receiving "250 OK" response from another email server. In which scenarios are we supposed to receive "550 Unknown user"?

• Also, after receiving "550 Unknown user" response, Rebex is sending RSET command. Is it valid to send reset command on receiving 550 error? Do we have any RFC references in detail answering these questions?

• Sample trace:

RCPT TO:dsupp1@mydomain.com
250 OK
RCPT TO:dsupp2@mydomain.com
250 OK
RCPT TO:xyz@mydomain.com
550 Unknown user
RSET
250 OK

Answer 1

• It is possible that two SMTP servers behaves differently.
  Are you using authenticated or unauthenticated session?

• Yes, it is valid to send RSET command in this case.
  RSET causes to abort current mail transaction, so the mail is sent to no recipient. This is the default behavior for Rebex SMTP - it is better to abort whole transaction than send a mail to only part of recipients.
  However, this behavior can be easily customized. Please check Handling rejected recipients article.

Fot SMTP see RFC 5321. For RSET command see section 4.1.1.5.

Comments

-Can you point out exact RFC reference which clearly mentions about when an mail server sends "550 Unknown user".
We are using both authenticated and unauthenticated modes.

-Also you said it is valid to send RESET and pointed out RFC, but didn't get clear idea on when it sends RESET and when to continue the transaction.

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1) This is from Section 3.3. - Mail Transactions:
If accepted, the SMTP server returns a "250 OK" reply and stores the forward-path.  If the recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible).

2) Regarding RSET specifics - what exactly is the question here?

3) I'm afraid that interpreting the RFCs is beyond the scope of Rebex support. The RFCs themselves are very clear. We would be happy to answer any questions you might have that are related to actual Rebex implementation, but it looks like your support contract has already expired. Please consider renewing it to enjoy continued support.