Binary package “qmail” in ubuntu xenial

a secure, reliable, efficient, simple message transfer agent

 qmail is meant as a replacement for the entire sendmail-binmail system on
 typical Internet-connected UNIX hosts.
 This package is based on the netqmail distribution of qmail, put
 together by a motley krewe of qmail contributors (see the README).
 It is derived from Daniel Bernstein's qmail-1.03 plus bug fixes,
 a few feature enhancements, and some documentation.