Good hint, thank you for that! Sadly, it seems to be more tricky than I was hoping. I did make an openssl client test connection, in order to retrieve the certificate. It seems that there's none.
openssl s_client -starttls smtp -connect myserver.mydomain:587 -showcerts
CONNECTED(00000003)
4007A44351700000:error:0A000126:SSL routines:ssl3_read_n:unexpected eof while reading:../ssl/record/rec_layer_s3.c:317:
no peer certificate available
No client certificate CA names sent
SSL handshake has read 323 bytes and written 363 bytes
Verification: OK
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
So it looks like tls is not configured at all, which is confusing after the log was basically just complaining
08.08.2024 15:35:46 Fehler node1 - Mail is unable to send (attempt 1), tls: failed to verify certificate: x509: certificate signed by unknown authority
However, if I disable STARTTLS in the account settings, the log will show
09.08.2024 08:40:53 Fehler node1 - Mail is unable to send (attempt 1), tls: first record does not look like a TLS handshake
I have to admit, I'm a bit puzzled now... either way seems to not work?