Mail configuration divides into three categories:
mail transfer agent (MTA): exim4, exim, postfix, sendmail, qmail, ssmtp, nullmailer, ...
exim4
exim
postfix
sendmail
qmail
ssmtp
nullmailer
mail utilities: procmail, fetchmail, mailx, crm114, ...
procmail
fetchmail
mailx
crm114
mail user agent (MUA): mutt, emacs+gnus, ...
mutt
emacs
gnus