MAIL FROM:<>RFC821, section 3.6 says:
This notification message must be from the server-SMTP at this host. Of course, server-SMTPs should not send notification messages about problems with notification messages. One way to prevent loops in error reporting is to specify a null reverse-path in the MAIL command of a notification message. When such a message is relayed it is permissible to leave the reverse-path null. A MAIL command with a null reverse-path appears as follows: MAIL FROM:<>RFC1123, section 5.2.9 says:
5.2.9 Command Syntax: RFC-821 Section 4.1.2 The syntax shown in RFC-821 for the MAIL FROM: command omits the case of an empty path: "MAIL FROM: <>" (see RFC-821 Page 15). An empty reverse path MUST be supported.(emphasis and links added by me). Error messages (in general: delivery status notifications) are sent this way, so they must not be rejected.
user@YOUR.DOMAIN
Next, someone from YOUR.DOMAIN sends an e-mail to that user at REMOTE.DOMAIN which gets forward to user@YOUR.DOMAIN That is, your server receives an e-mail with a local sender address and a local receiver address, which is relayed from a remote system (REMOTE.DOMAIN).
If you would reject those mails, the above scenario would break. Can you make sure that this scenario won't happen?
Other proposals what should be in this text? Send me an e-mail. Thanks.