Malformed MIME change flagging messages as spam

We've been using Campaign Monitor for a couple of years now with no issue, and every week we send out four or five newsletters for clients that are almost identical from week to week. (The text changes from week-to-week, but the HTML coding is exactly the same.)

A couple of weeks ago, we stopped receiving ALL messages sent through CM -- both previews and actual send-outs.

I discovered that all of our emails are now being caught by our internal spam appliance. (Symantec Mail Security Appliance, software version 7.7.0-17) The error that causes our messages to be flagged is "unscannable" due to a "Malformed MIME." We haven't made any changes to this appliance.

Given that we're using the same HTML code these past few weeks as we have for years, and that all our emails from multiple campaigns are suddenly experiencing the same "Malformed MIME" issue, I have to assume it's something on Campaign Monitor's end.

Could someone help shed light on this issue?

Ben Ben, 7 years ago

Hi Gary,

We are looking into this issue and think it might be to do with a change we made recently which wraps headers to 76 characters - having them longer than 76 characters shouldn't be a problem but it triggers a flag in spamassassin.

We're rolling back this change and will be deploying this on Sunday afternoon.

I'm sorry for the inconvenience Gary. It's tricky when we're trying to fix an issue with one spam filter and it triggers another!

And I should point out that we'll only partially be rolling back the solution for the subject line, and not the body which will continue to use the new code to make sure the spamassassin flag isn't triggered (which only happens when you use long custom fields in your campaign).

Ben Ben, 7 years ago

Gary, as I mentioned in your support request I'm going to send you a few emails from our development environment to see how your filter reacts to the changes.

Ben Ben, 7 years ago

Just to confirm, this has been deployed and fixed.

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