How to authenticate to the server over a secure connection to send mail from a script?

Question

My current script uses this simple command:

mail -s "$my_msg" username\@mycompany.com </dev/null

When changing it to use a mailing list rather than the hardcoded username:

mail -s "$my_msg" mailing_list_1\@groups.mycompany.com </dev/null

It produces:

550 Unrouteable address

I need to be able to authenticate to the server over a secure connection. How do I do this?

Linux version 2.6.26