Easyform always in spam(?)

I have a few sites where I switch the default contact form to a new Easyform, usually with just another field or two.
Every time, the mail end up in the spam folder after the switch. (then I have to instruct the users on how to modify there spam filter).
Any suggestions on why this happens (the sender, the subject etc are all the same)

Is there any information in the header of the received email why it was filtered?

I can not really see any difference, except that the subject is now:

Subject: =?utf-8?q?Subject?=

It should probably be
Subject=Subject

This is just an valid encoding thing, see RFC 2047 https://tools.ietf.org/html/rfc2047#section-5 (there are also examples in section 8). I don't think it will influence any spam filter.

Best you take both raw message before sendout (Products.PrintingMailHost is your friend) and diff them?