Ssmtp

smtp
ssmtp - "ssmtp is a send-only sendmail emulator for machines which normally pick their mail up from a centralized mailhub (via pop, imap, nfs mounts or other means)"

ssmtp - "extremely simple MTA to get mail off the system to a mail hub. A secure, effective and simple way of getting mail off a system to your mail hub. It contains no suid-binaries or other dangerous things - no mail spool to poke around in, and no daemons running in the background. Mail is simply forwarded to the configured mailhost. Extremely easy configuration."

Python Alternative
See ksmtp

Installation
Redhat based: yum install ssmtp
 * 1) in EPEL repo

Debian based: apt-get install ssmtp

Replace sendmail: mv /usr/sbin/sendmail /usr/sbin/sendmail.original ln -s /usr/sbin/ssmtp /usr/sbin/sendmail

Test: echo "hello world" | mail -s "test" kenneth@demo.oeey.com

Configuration
Configuration file: /etc/ssmtp/ssmtp.conf

Configuration man page: man ssmtp.conf

Gmail
To relay mail though a gmail user: AuthUser=[USERNAME]@gmail.com AuthPass=[PASSWORD] FromLineOverride=YES mailhub=smtp.gmail.com:587 UseSTARTTLS=YES

To force all mail to a specific user: Root=[EMAIL] AuthUser=[USERNAME]@gmail.com AuthPass=[PASSWORD] FromLineOverride=YES mailhub=smtp.gmail.com:587 UseSTARTTLS=YES

keywords
linux smtp smarthost email mail