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.