Chris Mahoney wrote:

Out of interest, how "should" a mail client issue a rejection?

Rejections should be performed by the server hosting the mailbox during the
SMTP handshake at the HELO or EHLO command or, possibly, the MAIL TO
command. If the host named in those commands does not pass the accepting
server's tests, a rejection error (550 is the number, I believe) is issued
to the connecting router.

If a client were to issue a rejection, it would do so in a similar manner,
but the problem is that the rejection would then be to the client's mailbox
server. What good would that do? In fact, I don't see how it could ever
happen, since it's the client that initiates the connection, not the mail
server, and it's using POP, not SMTP protocols.
--
Brian Tillman