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[ietf-smtp] Mail forwarding to Gmail problem/question

2016-02-03 04:35:16
Hi, I just wondered if anyone here had any suggestions for this issue we constantly have...

We provide a paid SMTP relay service to some of our customers who have ISPs with awful SMTP relay services.

One constant issue we have is that several of our customers insist of setting their internal mail servers up to forward all mail for certain users back out to gmail addresses. That means that they send bucketloads of spam (incoming spam to them, which then gets forwarded back to our servers). What then happens is that Google (reasonably understandably) start limiting our servers from sending mail to them, and they even seem to start rejecting legitimate mail just because it's come through our servers which are also sending 'spam' to them.

We do tell the customers not to do this. They do it so they can get their mail at home, so we tell them it's probably better to simply allow access to their internal mail server from the Internet (they have IMAP4, so it's a nice user experience as well), but they still want to forward the mail back out again regardless.

What I've started doing is detecting this indiscriminate forwarding from the customers' MTAs and route that mail to one particular MTA which is only used for sending the forwarded messages on to Google. Hopefully that will limit the 'bad reputation' to just that one server's IP address.

We could start passing the outbound mail through our own spam filter as well, but that's something we're reluctant to do because it'll lead to false positives.

Any other ideas or thoughts?

Am I right in interpreting Google's behaviour, or are they clever enough to detect MTA forwarded messages and not assign poor reputation to the forwarding MTA in that case?

Is there something clever we should be doing to tell Google what's happening?

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