After upgrade of Linux distribution Courier stopped to accept emails delivered over TLS or SSL.
There was just nice error message in the log file:
couriertls: /etc/courier/esmtpd.pem: error:0906D06C:PEM routines:PEM_read_bio:no start line
Long story short. The problem was in pem file.
Previous versions of Courier-SSL were able to read files with Windows EOL. The new version is failing with this nice error.
Solution is simple: get rid of Windos EOL.
You can use e.g. dos2unix
Restart services and everything will work 🙂