Thank You, Pete. I've done that and, after backing up the original, regenerated the semdmail.cf file with:
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
But I get the following.
<<< 501 Validating Sender
554 5.0.0 Service unavailable
<<< 503 Bad sequence of commands
I know I have to authenticate through POP3 before sending but how could I go about setting up authentication with sendmail?