Yesterday evening I completed for 50% an entry from my wish-list regarding my servers. On the backup-MTA I now have TLS and authenication enabled on Sendmail
I wanted to have this enabled, because from my laptop I often setup an SSH-tunnel, but on other devices it wasn't really possible
After some Google-ing I found this page which was really helpfull in setting the stuff up on sendmail. The next step is to use TLS between the MTA's I own and TLS between other parties who provide TLS on their MTA.