Jul 11 15:20:58 tor sendmail: r6AKjOD07: to=
mailserver.com was the hostname of the server, sendmail sends this by default and many mailservers will reject mail to a hostname that does not resolve or exist.
The easiest way is just to change the hostname and make sure it does resolve to something.
I read there is a way in sendmail.mc to manually set a hostname but I never got it working:
The first thing you need to remember is not to check from the same host/server itself. This is a silly mistake I made, the reason is that many mailservers and especially postfix are configured to allow relaying from the localhost/same host. If you do that you'll get a false positive.
*Make sure you test from another host/system than the mail server itself!
telnet yourmailserverhost.com 25
220 Courier (FreeBS........