powerful, efficient, and scalable Mail Transport Agent (documentation)

 Sendmail is an alternative Mail Transport Agent (MTA) for
 Debian. It is suitable for handling sophisticated mail
 configurations, although this means that its configuration
 can also be complex.
 .
 Don't try anything fancy in sendmail without using this for guidance.
 .
 Because of the complexity, you may find yourself wanting to peruse
 the documentation files included herein. The main files are:
   cf.README.gz -- A reference for all things in sendmail.{mc,cf}
   op.{ps,txt}.gz -- The care and feeding of Sendmail.
   faq.txt.gz -- Sendmail FAQ, current as of the package date.
 .
 Other files include the references for smrsh and libmilter.