Seemingly eligible mails not getting DKIM signed

Hello and long time (4 years) no see,

The vast majority (3 or 4 9s) of mails here get correctly DKIM signed,
provided the domain in question is actually set up for this.

The ones that don't fall into 2 categories:

1. Ones with broken header From: (binary junk directly next to the
   domain for example), which result in an empty dkim_domain and thus no
   signage. Not my problem and easy enough to detect and potentially fix.

2. Mails that don't get signed despite having a valid dkim_domain set.
   Looking at the mail data there seems to be nothing amiss and rspamd
   also reports that envelope and header From are identical and has
   nothing bad to say about header fields. 
   I have "dkim_strict = 0" since I don't really want to defer mails
   needlessly, never mind that whatever is causing this is consistent with
   the handful of user/accounts/addresses where this happens, so deferring
   would serve no real purpose here.

There is nothing in the logs of course either.

Any ideas what could cause this, i.e. some header encoding (this is Japan
after all) that Exim is silently failing on and decides not to sign?

Any means of getting more info without running production servers in debug
mode?

Regards,

Christian
-- 
Christian Balzer        Network/Systems Engineer & Architect               
[email protected]   	Rakuten Mobile Inc.

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