Quoting Brian May (brian@linuxpenguins.xyz):
msmtp is interesting implementation of sendmail, because it lets individual users store their remote smtp servers in ~/.msmtprc.
The category of maximally simple, stripped-down utilities to do outbound SMTP is collectively called 'nullmailers', after the name of the earliest best known example. I catalogue them here: 'Nullmailers' on http://linuxmafia.com/kb/Mail/ Note that some of them can do modern crypto extensions, most sadly not. Ordinarily, I strongly favour full-service MTAs = SMTP daemons (Exim4, Postfix, Courier-MTA), but there's one host where I opted for a nullmailer anyway: Silicon Valley Linux User Group's dedicated Web server www.svlug.org has no SMTP functionality running on it (those being on separate machine lists.svlug.org), but it's still desirable to send root's mail to someone offsystem so that an admin gets notified of anything serious in the logs, etc. So, nullmailer (does that job minimally, doesn't add much to the public attack surface).