To configure postfix for usage with t-online, I am using the very same settings
since decades.
However, with leap-42.1 this does not work any more. The issue has readily been
pointed out by Okaf Hering in a mail discussion on opensuse-packaging in August
2015 (Thread - start on 2015-08-04 / 11am)
The problem is the fact that the entry

smtp_tls_cert_file = /etc/ssl/

causes TLS to fail as there is nothing usable to be found at this location
(apparently).

Commenting this file out makes mail transmission work again.

Therefore, please fix - this used to work for years.

By the way: without deinstalling systemd-logger and installing syslog-ng I had
never ever found the issue, as journalctl only supplies a very reduced amount
of information - and is hard  to seek.
This is - IMHO - definitively a step back. It took me hours to get an useful
debugging information.