Hey there!
First off, Welcome to Linux!
Unfortunately, I have no solution for your sendmail problem except to suggest switching over to
Qmail. Sendmail is a very large, very old, very difficult to use package. Especially if you're just learning Linux, I would definitely recommend not using sendmail if at all possible.
Sorry I can't help with your problem, but perhaps you will be able to switch to Qmail.
Cheers!