Estes passos funcionaram
Etapas:
vim /etc/mail/sendmail.mc
no final:
define('MAIL_HUB', 'example.com.')dnl
define('LOCAL_RELAY', 'example.com.')dnl
e depois:
sendmailconfig (or /etc/mail/make depending on your distro)
service sendmail restart
teste:
echo -e "To: [email protected]\nSubject: Test\nTest\n" | sendmail -bm -t -v
echo -e "To: user\nSubject: Test\nTest\n" | sendmail -bm -t -v
daqui: link