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.