Setup mail forwarding in postfix on Debian or Ubuntu

Install Postfix
$ sudo apt-get install postfix

After Install check postfix status
$ sudo service postfix status

Further check that postfix is running a server on port 25 with the netstat command

$ sudo netstat -ltnp | grep 25

Output : 

tcp        0      0 127.0.0.1:25      0.0.0.0:*    LISTEN      1926/master     
tcp6       0      0 ::1:25            :::*         LISTEN      1926/master

Verify DNS settings of domain

$ sudo dig example.com mx

Configure postfix to forward mails

Check the path of config folder using  postconf command

$ postconf | grep config_directory

config_directory = /etc/postfix

Edit the main.cf file and add following lines at end if  it

virtual_alias_domains = mydomain.com anotherdomain.com

virtual_alias_maps = hash:/etc/postfix/virtual

Now Create /etc/postfix/virtual file and add emails to forwad along with destination mails like:

myemail@domain.com   destination@gmail.com

othermail@domain.com otherdes@yahoo.com

First mail  address receive emails, and the second address would forward the emails.

For all forward emails use following lines

@mydomian.com  des@gmail.com 

After this save file and close it and run following commands

$ postmap /etc/postfix/virtual

$ sudo service postfix reload.

Now try to send an email from somewhere to the address on yourdomain and the same mail forwarded to other account.

Install and Configure Mail Server on Linux System

Mail Transfer Agent
A Mail Transfer Agent (MTA) is the program which receives and sends out the email from your server, and is therefore the key part. The default MTA in Ubuntu is Postfix, but exim4 is also fully supported and in the main repository.
Postfix - this guide explains how to set up Postfix.

In order to install Postfix with SMTP-AUTH and TLS, first install the postfix package from the Main repository using your favorite package manager. For example:
sudo aptitude install postfix


Configuration
From a terminal prompt:
sudo dpkg-reconfigure postfix
Insert the following details when asked (replacing server1.example.com with your domain name if you have one):
General type of mail configuration: Internet Site
NONE doesn't appear to be requested in current config
System mail name: server1.example.com
Root and postmaster mail recipient: <admin_user_name>
Other destinations for mail: server1.example.com, example.com, localhost.example.com, localhost
Force synchronous updates on mail queue?: No
Local networks: 127.0.0.0/8
Yes doesn't appear to be requested in current config
Mailbox size limit (bytes): 0 
Local address extension character: + 
Internet protocols to use: all