On 19 Apr 2019, at 20.04, Martin Müller via dovecot <dovecot@dovecot.org> wrote:

2019-04-19 18:53:54 imap-login: Info: Login: user=<office@ourdomain.at>, method=PLAIN, rip=80.75.xx.35, lip=136.xxx.9.172, mpid=28364, TLS, session=<vDTC9uSG2opQS/Yj>

All 4 different MUAs Thunderbird are logged in the same way. They are behind a router, so they having the same remote IP. 
So I cant differentiate, which MUA causes which event.

Is there a way, to identify which client raise a special event?

I think Thunderbird does send IMAP ID information so you can try adding "imap_id_log = *" to your config to get the info logged.

Maybe it has some information to identify the different clients.

Sami