There are two key differences such routed mails have over directly received mails:
- The IP address of the incoming mailer connection will not match the mailer information of the domain listed on the MAIL FROM: line in the SMTP conversation.
- The mail itself will have an additional
Received-By: header in it from the relaying mailer.
The first point is critical when it comes to anti-spam, since a lot of AS technologies focus around discarding email not coming from where it should (see also, SPF) or coming from IP addresses that are funny looking (IP reputation). If you are receiving relayed, your AS systems must not consider IP address as part of their checking.
It works like this:
- The internet-facing mailer at example.client sends a message to example.yourcorp via the MX record.
- The server listed in your MX record, which is off of your network, receives the connection from mailer.example.client. It looks like it comes from example.client, doesn't smell overly processed, and forwards it on to mailer.example.yourcorp.
- Your mailer.example.yourcorp receives an incoming message from example.client, but sent from example.antispam instead.
If this were the Internet circa 1992, that wouldn't be a problem. It was a more trusting time back then, and in that case mailer.example.yourcorp would merrily accept the message and no one would be the wiser.
Spam throws the monkey-wrench in here. At this point, an anti-spam service running directly on mailer.example.yourcorp could throw a fit. Since example.client's SPF record (for example) doesn't state that example.antispam is an authorized mailer, it could drop the message on the floor never to be seen from again.
Anti-spam services work best when they're running on the mailer that directly receives mail from the general internet. This is in large part due to the fact that IP reputation services have been one of the best anti-spam technologies, and to utilize it you need to see those TCP connections. Hide behind a mail relay, and you lose that advantage.
The second point is a by-protocol addition to the mail headers required by the SMTP standards. The client should never notice.