stat=Deferred: 451-Non-existent domain mailserver.com
Jul 11 15:20:58 tor sendmail[9617]: 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:
vi /etc/mail/sendmail.mc
define(`confDOMAIN_NAME', `mail.yourdomain.com')dnl
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