E-Mail to non-existent account not returning undeliverable message

Answer

There are four options that may be used when mail is received for an unknown user

1. Fail to user account
2. Redirect to mail server
3. Reject the mail
4. Transfer to user account

Option 3 will result in the mail being rejected as to an unknown user within the SMTP delivery phase of the message transaction. This is the only option that will result in an undeliverable message being returned to the sender.

With any of the other options the mail is actually delivered to a mailbox, either locally with options 1 and 4, or passed on to a remote server with option 2.

Keywords:domain, unknown user, undeliverable, mesage, failure