I built my mailserver on a vserver after the ISPmail tutorial (https://workaround.org/ispmail/wheezy) (which is awesome). The only remaining problem is that it does not seem to use TLS with outgoing mail according to websites such as https://ssl-tools.net/mails and other checking tools. My actual problems is that I do not know where to search for more revealing logs because mail.log is not very helpful in this case (when sending out a mail offering starttls and it just does not get accepted).
What does work: sending and receiving mail with various clients, using the cert from startssl for webserver HTTPS, DNS + reverse DNS, IPv6.
What could be a problem: the servers hostname is xy. I have domain.de and xy.domain.de in the cert while xy.domain.de does not exist in DNS. (Domain reseller giving me only the SLD and sells the feature "subdomains" only in the "build your shiny webserver" kit). But domain.de points to the server IP and vice versa.