Hello @BeWog,
I had the (maybe) same problem long time ago, when I was setting up my server. If I remember right I got it working by doing the following:
Trying the settings in Thunderbird, send and receive email
Adding globalSettings__mail__smtp__trustServer=true as you mentioned
Switching to port 587 as 465 did not work for me.
In my case I had a self hosted email server. If you have an external email server like gmx (in Germany) you need to enable the ports before you can use them (You will find out if you test it with thunderbird).
I saw others topics speaking about spam.
I had a doubt about the OVH server so I made multiple test with 3 differents emails to understand.
I also had 2 differents ways to use Python script (smtplib.SMTP_SSL and smtplib.SMTP with tls) and i didn’t understand immediatly but it depend of the port - 465 (SSL) or 587 (TLS).
Of course, objective is security so, I do not use globalSettings__mail__smtp__trustServer