[Rspamd-Users] rspamd DKIM, Mimecast and Content-Encoding

[Florian Effenberger]

Hello,

lately, I've been struggling to send a message to a domain that recently 
switched to Mimecast. I get DKIM rejections for the majority, but not 
for all messages. Clients are mostly Thunderbird and Roundcube.

I consider my setup pretty much standard (DKIM signing in rspamd via 
Postfix milter, no other filters that change mail content), and all 
major e-mail providers and half a dozen of DKIM/DMAR testing sites 
confirm the signature is proper.

I now tested it with aboutmy.email and get errors on the message body 
signature, but only if the Content-Transfer-Encoding is 8bit. If I 
switch to quoted-printable, it works just fine - and I can confirm I can 
deliver to the Mimecast-enabled domain in this case as well.

My dkim_signing.conf and my arc.conf contain this:
(FreeBSD, but I have the same problem on Debian with the rspamd-Repo)

path = "/usr/local/var/lib/rspamd/dkim/$selector.key";
selector_map = "/usr/local/etc/rspamd/dkim_selectors.map";
allow_username_mismatch = true;
try_fallback = false;
use_esld = false;

For years I never had any deliverability issue, rspamd is just rock 
solid for me, so I wonder whether I miss something here... anyone has 
any advice where to start looking for?

Thanks a lot
Florian