/var/log/exim4/mainlog is just fine.
however as said above the entry you already posted seems to be from something that’s already stuck in the queue and probably won’t go out anyway anymore. it sadly tells nothing about, why it got into the mailqueue in the first place.
a full entry of a mail while trying to be sent the first time will be more than these three lines, holding informations about the sender, the recipient, the server connected and it’s response.
see f.i. @brackenhill-mob post/logentries in #9 above … [SOLVED] Upgrade to 1.2.0 borked Exim SMTP to GMail/GMX/Yahoo/others
there you can see a response from the smtp server complaining about the helo entry etc. - if you don’t have that your problem lies somewhere else.
but if we want to see why your mails get stuck, we need to get these informations, hence my suggestion of sending out an email that is expected to not work properly and immediately track the according log entries to that request.
in other words: only if you can reliably reproduce an issue you can track it down properly