# dpkg -l|grep ssmtp
iF  ssmtp                                  2.64-4fakesync1                                 extremely simple MTA to get mail off the sys

if ssmtp is not installed yet, install it by using

# apt-get install ssmtp

If you are using Gmail account , change the root, authuser, and authpass options to the email address, username, and password of your email account. If you’re using another mail service you’ll need to change mailhub to the relevant SMTP server.

edit /etc/ssmtp/ssmtp.conf and add these lines

--------------------------------------------
root=alert@example.com
mailhub=smtp.example.com:587
rewriteDomain=
hostname=smtp.example.com:587
UseSTARTTLS=YES
UseTLS=YES
AuthUser=alert@example.com
AuthPass=password
AuthMethod=LOGIN
FromLineOverride=YES # optional
--------------------------------------------

Change the ‘From’ text by editing /etc/passwd to receive mail from ‘root@server1.example.com’ instead of just ‘root’.
# chfn -f root@server1.example.com root

edit /etc/ssmtp/revaliases and add this line or add more line for another user
--------------------------------------------
# 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:alert@example.com:smtp.example.com:587
--------------------------------------------

to test it

# echo “test send email using ssmtp|mail -s “TEST” alert@example.com

once you are able to receive that test message, proceed to edit /etc/hosts.allow and add these 2 lines, so you will be notified if someone login to your sever

--------------------------------------------
SSHD: ALL: spawn (/bin/echo "SSH connection to %H from %h[%a]" | \
/usr/bin/mail -s "SSH Login Alert [server1.example.com]" alert@example.com)
--------------------------------------------