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=YOUR_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=YOUR_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