[Mimedefang] Postmaster, <>, and a Backup MX

[Troy Carpenter]

David,

After reading all the posts on the rfc-ignorant situation, I agree with
your stance on the issue, but...

I just came across an interesting scenario that may not pass muster with
your test.  I have a backup MX for my domain.  After investigating some
interesting entries in my maillog, I found that my backup MX did something
interesting before trying to send email to my server.

It turns out that when the backup MX is receiving mail for my domain
(either because my server is down or a spammer bypassed my primary MX),
the backup MX would open an SMTP session using <> in the MAIL FROM: line,
then use the receiver name in the RCPT TO: line to verify the recipient
before accepting the email.  The original session would then be closed.
(My guess is that they do that because many mail servers disable the VRFY
command)

So, if I used the same logic on my site as yours, then a legitimate email
addressed to postmaster at carpenter.cx, relayed through my backup MX (for
whatever reason), would be rejected because of the way the backup MX would
verify that the account postmaster existed.

Granted, this is a contrived situation, but it may make others aware of
situations they need to consider before implementing the same policy as
yours.

Troy Carpenter
troy at carpenter.cx