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

[Florian Effenberger]

Hi,

Vsevolod Stakhov wrote on 21.10.24 at 18:02:

> Mimecast might still not support 8bitmime, so there could be some glue 
> there. WRT Rspamd changes - there was ARC signing break in 
> 3.10.0..3.10.1 that has been fixed in 3.10.2: https://github.com/rspamd/ 
> rspamd/issues/5173

thanks a lot for your reply!

I discussed this also over at mailop, and it seems somehow indeed be 
related to the 8bit encoding that both Thunderbird and Roundcube produce 
by default. Postfix accepts the mail, DKIM signing takes place, and when 
delivering the mail, it seems to get converted to quoted-printable. That 
seems to break the signature - interestingly it is reproducible neither 
on dmarctester.com, dkimvalidator.com, and I don't have any problem to 
send mail to the larger ISPs. Also Google Mail shows the message with a 
valid signature. I can reproduce the problem so far only with Mimecast, 
and with aboutmy.email. I can even send the same mail to Mimecast, Bcc 
my GMail account - Mimecast bounces, GMail shows a valid signature.

What puzles me a lot is where the problem comes from. I consider my 
setup pretty much standard here - I connect to my MX, which sends out 
the message that it routes through rspamd (3.10.2 from the official 
repo), which takes care of DKIM signing. No other modification of the 
mail takes place, no other filter, milter or proxy is in between, no 
other smarthost, I directly send the mail.

Postfix has SMTPUTF8 disabled because of Dovecot-LMTP, that is the 
unique setting on encoding I configured.

That's a setup that's probably in wide use out there. Mail clients in 
use are Thunderbird and Roundcube, also that is pretty much standard.

Sorry for the noise though - I am not sure anymore this is a problem 
with rspamd as I had initially assumed. Happy to provide more details of 
course, if it helps to chase a bug - but as said, not sure anymore that 
is the case...

Florian