I am getting closer to localize the problem. Now I can reliably reproduce the DKIM failure - it is enough to send an email from Gmail with the body size (may include an attachment) bigger than 65536 bytes (the size of an unsigned short integer).
I see big emails with DKIM coming from other domains without any error though, which seemingly indicates there is a problem at Gmail, or during the transit, but when I test the Gmail message with Perl Mail::DKIM::Verify, then it passes perfectly OK, so it looks more like an Exim bug. I am doing some more test, so I hope I'll manage to narrow it a bit more.
I would appreciate if others could try sending themselves an email from Gmail bigger than the threshold, and told me whether they get the same result (bodyhash mismatch). If not, then it might be a platform dependent problem.
You could also try sending 65kB+ emails from other DKIM enabled domains. Just keep on mind that in the standard setup, Exim won't check DKIM's sent locally (from your own domain). If you do not have several servers, you can send such messages to me.
