Setup mail forwarding in postfix on Ubuntu or Debian

Postfix mail forwarding
Postfix is a commonly used MTA (Mail Transfer Agent) program that can receive, deliver or route emails. So if you have your own domain and a server then using postfix you can setup emails on the domain like
Setting up a full featured mail server with postfix is a lengthy process requiring quite some effort. Furthermore, using your own mail server you do miss out the convenience and features of the popular free email services like gmail or hotmail. One way is to use google's paid service to integrate gmail with the domain email.
Alternatively, setup your webserver to forward all mails coming on your domain email, to your other address like gmail. Doing so would allow you to accept mails on your personal domain emails, making it look professional and at the same time use gmail service to access the same and enjoy the convenience of gmail too.
mail for ==> [ Postfix server ] ==> forwarded to
So this post shows how to setup mail forwarding on a Debian or Ubuntu server using the postfix program. For other distros like CentOS and Fedora too, the steps are nearly the same.

Install Postfix

Installation is super quick. Postfix is there in the default repos, so just use apt-get to install.
$ sudo apt-get install postfix
During installation it would ask a couple of questions about how to configure the mail server. Fill in the required information and proceed. Once the installation is complete, the mail server daemon would be started.
$ sudo service postfix status
 * postfix is running
You can further check that postfix is running a server on port 25 with the netstat command
$ sudo netstat -ltnp | grep 25
tcp        0      0  *               LISTEN      1926/master     
tcp6       0      0 ::1:25                  :::*                    LISTEN      1926/master
The last column shows the pid of the processing, which is postfix here.

Verify DNS settings of domain

Before proceeding to configuring postfix to forward mails on behalf of your domain, its necessary to ensure that the MX records for your domain are pointing to the right server. This can be done quickly with the dig command. Run it from your local machine.
$ dig mx

; <<>> DiG 9.9.3-rpz2+rl.13214.22-P2-Ubuntu-1:9.9.3.dfsg.P2-4ubuntu1.1 <<>> mx
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- 08:59:17="" 0="" 10="""" 1="" 2014="" 28="" 4096="" 65="" 7230="" 74="" 85100="" additional:="" answer:="" answer="" authority:="""" edns:="" flags:="" fri="" id:="" in="" ist="""" mar="" msec="" msg="" mx="" noerror="" opcode:="" opt="" pre="" pseudosection:="" qr="" query:="" query="" question="" ra="" rcvd:="" rd="" section:="" server:="" size="" status:="" time:="" udp:="" version:="" when:="">
In the 'ANSWER SECTION' it can be seen that is reported as the mail server for Next, dig up the a records for the domain and it would show the server ip to which it points.
$ dig a

; <<>> DiG 9.9.3-rpz2+rl.13214.22-P2-Ubuntu-1:9.9.3.dfsg.P2-4ubuntu1.1 <<>> a
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- 09:00:19="" 0="""""" 1="" 2014="" 28="" 30="" 4096="" 43356="" 65="" 85045="" a="" additional:="" answer:="" answer="" authority:="" edns:="" flags:="" fri="" id:="" in="" ist="""" mar="" msec="" msg="" noerror="" opcode:="" opt="" pre="" pseudosection:="" qr="" query:="" query="" question="" ra="" rcvd:="" rd="" section:="" server:="" size="" status:="" time:="" udp:="" version:="" when:="">
Now the ANSWER SECTION shows the ip address of which over here is correct. Do the same for your domain and verify that the mx records are setup correctly.

Configure postfix to forward mails

Now comes the final task of configuring postfix to forward mails, and it is surprisingly simple. Find the path of the configuration directory using the postconf command
$  postconf | grep config_directory
config_directory = /etc/postfix
Now we know that all the necessary configuration files lie in the above directory, so get inside it. Edit the file
And add the following lines at the end of it
virtual_alias_domains =
virtual_alias_maps = hash:/etc/postfix/virtual
The first line virtual_alias_domains lists the domains, for which postfix is going to accept emails. Multiple domains are added separated by a space.
The second line virtual_alias_maps specifies the path to the file which is going to contain mappings specifying how to forward emails for the above domains.
Now open the /etc/postfix/virtual file (create one if it does not exist) and add to it the emails you want to forward along with the destination emails.
# lets forward 2 emails
The first email is the address on which postfix shall receive emails, and the second is the address where postfix would forward the emails.
The mail can be forwarded to multiple destinations
To catch and forward emails to any address for a given domain, use the following notation
# forward all emails
After entering the forwarding rules, save the file and close it. Then run the following commands for the new settings to take effect
Update the postfix lookup table
$ postmap /etc/postfix/virtual
Reload postfix configuration
$ sudo /etc/init.d/postfix reload

# or

$ sudo service postfix reload
Check with the postconf command that the domain aliases and alias file have been setup properly.
$ postconf -n | grep virtual
virtual_alias_domains =
virtual_alias_maps = hash:/etc/postfix/virtual

Test it now

Now try sending an email from somewhere outside the world, to the address on your domain, and you should see the same mail forwarded to the gmail account.