Binary package “opensmtpd” in ubuntu eoan

secure, reliable, lean, and easy-to configure SMTP server

 The OpenSMTPD server seeks to be
  * as secure as possible, and uses privilege separation to mitigate
    possible security bugs
  * as reliable as possible: any accepted email must not be lost
  * lean: it covers typical usage cases instead of every obscure one
  * easy to configure, with a configuration syntax reminiscent of the OpenBSD
    Packet Filter's (PF)
  * fast and efficient: it can handle large queues with reasonable performance