I've used sendmail to handle email from JNOS BBS systems to/from the internet back 'in the day'. I don't recall that it was particularly user friendly, but if you could get a JNOS BBS going, sendmail was a piece of cake!
I used to be one of the sysops for an emergency communication system for 12 hospitals. Voice and data... the latter via JNOS running on Slackware Linux. I still have all of the configuration files somewhere, but I doubt that I'd remember half of the stuff I used to know so well!