There are times when you host your mail on another server like google mail ..
To do so, you may set it in the configuration to set sendmail to redirect mails to outside instead of identifying it as local recipient.
Note: do not miss the 'dot' behind the domain.
then compile with m4 in /etc/mail
m4 sendmail.mc > sendmail.cf
restart sendmail service:
sendmail -v firstname.lastname@example.org < /dev/null
check your email, and makesure to check your spam folder as this is going to be an empty email, and spam filtering doesn't like that ;)