relay

The below is specific to CentOS-5.4 and may work similarly with other distros.

• Additional packages required if not installed already:
  

  sendmail-cf
  m4
  make
  cyrus-sasl-plain

• Edit ”/etc/mail/sendmail.mc”:
  
  define(`SMART_HOST', `{smtprelay.domain.tld}')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo.db')dnl
• Create file ”/etc/mail/authinfo” with below contents and chmod 640:
  
  AuthInfo:smtprelay.domain.tld "U:{username}" "P:{password}" "M:PLAIN"
• Update the sendmail conf and db hashes:
  
  cd /etc/mail
make
• Restart sendmail for the new configs to pick up.
  
• Now mails sent to localhost is relayed via your SMTP provider.