Send system mail via external smtp server (on Debian based systems)

Install ssmtp

apt-get install ssmtp

/etc/ssmtp/ssmtp.conf

# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
root=RECEIVER_EMAIL_ADDRESS

# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
mailhub=SMTP_SERVER

# Where will the mail seem to come from?
#rewriteDomain=

# The full hostname
hostname=YOUR_HOSTNAME

# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address
FromLineOverride=YES

AuthUser=YOUR_MAIL_USERID
AuthPass=YOUR_USERID_PASSWORD
UseTLS=YES
UseSTARTTLS=YES

/etc/ssmtp/revaliases

# sSMTP aliases
#
# Format:       local_account:outgoing_address:mailhub
#
# Example: root:your_login@your.domain:mailhub.your.domain[:port]
# where [:port] is an optional port number that defaults to 25.
root:sender@mydomain.com:mail.mydomain.com:587

Test eMail:

ssmtp root
Subject: test
test
^d