After searching throught the web, I eventually found a simple way to send emails via linux terminal, by using a simple package called Postfix. This guide is tested with Ubuntu 12.04.I use a Gmail account as SMTP server on Ubuntu-Linux server.

Reason as to why I needed to be sending mails via terminal was for system checks, Server disk space monitoring, Server ping status and etc.

Postfix via smtp.gmail.com.

First, install all necessary packages:

$sudo su
$apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules

If you do not have postfix installed before, postfix configuration wizard will ask you some questions. Just select your server as Internet Site and for FQDN use something like mail.example.com

Then open your postfix config file:
$nano /etc/postfix/main.cf

and add the following lines and the end of it(ctrl+shift+c and ctrl+shift+v):

relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes

create file and add username and password

$nano /etc/postfix/sasl_passwd

add this line and edit

[smtp.gmail.com]:587    USERNAME@gmail.com:PASSWORD

Fix permission and update postfix config to use sasl_passwd file:

$chmod 400 /etc/postfix/sasl_passwd
$postmap /etc/postfix/sasl_passwd

Next, validate certificates to avoid running into error. Just run following command:
$cat /etc/ssl/certs/Thawte_Premium_Server_CA.pem | sudo tee -a /etc/postfix/cacert.pem

Almost there,now reload postfix config for changes to take effect:

$ /etc/init.d/postfix reload
$ exit

Testing

Checking if mails are sent via Gmail SMTP server

If you have configured everything correctly, following command should generate a test mail from your server to your mailbox.

$echo "Test mail from `id -nu`@`hostname`" | mail -s "Postfix Test Success" xxx@gmail.com

Make sure the 2-factor authentication is Disabled on Gmail.