Outgoing mails are usually send via SMTP to an MTA (Mail Transfer Agent) which is one of Postfix, Exim, Sendmail or OpenSMTPD on UNIX/Linux in most cases. You really don't want to setup and maintain any of those full blown kitchensinks yourself because they are complex, fragile and hard to configure.

My use case is simple. I need to send automatically generated mails from cron via msmtp/sSMTP/dma, mails from various services and network printers to GMail without giving away my GMail credentials to each device which produces mail.

Main features

Supports incoming SMTPS/TLS (465), STARTTLS (587) and unencrypted SMTP (25)
Checks for sender, receiver, client IP
Authentication support with file (LOGIN, PLAIN)
Enforce encryption for authentication
Supports outgoing STARTTLS (587) and unencrypted SMTP (25)
Forwards all mail to a smarthost (GMail, MailGun or any other SMTP server)
Small codebase (smtp-proxy ~250 LoC, chrj/smtpd ~1200 LoC)
IPv6 support