Sendmail Installation & configuration on centos with virtual user and domains

The Sendmail program is a very widely used Mail Transport Agent (MTA). MTAs send mail from one machine to another. Sendmail is not a client program, which you use to read your email. Sendmail is a behind-the-scenes program which actually moves your email over networks or the Internet to where you want it to go.. 
This guide show you have to install and configure sendmail with virtual user and domains - 
Installation  -

Check if sendmail is installed or not:

   # rpm –qa | grep sendmail 

If it is not installed, then install the sendmail package using following command:

   # yum install sendmail sendmail-cf

By default sendmail listen on local interface i.e 127.0.0.1

To configure the sendmail to listen to all interface at the host, just comment the following line in “sendmail.mc”:

# vi /etc/mail/sendmail.mc

     From:
     DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

     To
     dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

Accepting mail for virtual domains/users
1.Create a new system user and set the password for that user.

useradd exampleuser passwd exampleuser

2. Add the user to the sendmail’s virtusertable file. Open the ‘/etc/mail/virtusertable’ file and add the following:

exampleuser@exampledomain.tld exampleuser

Each line in the /etc/mail/virtusertable file should be of the following format:

username@domainname systemuser

3. Add each new domain name to the /etc/mail/local-host-names file

exampledomain.tld

4. Generate a new sendmail.cf file by using the following commands:

cd /etc/mail make all /etc/init.d/sendmail restart

Rewriting outgoing email addresses

1. Add the following lines to the /etc/mail/sendmail.mc file:

1 2 3

FEATURE(masquerade_envelope) FEATURE(genericstable, `hash -o /etc/mail/genericstable') GENERICS_DOMAIN_FILE(`/etc/mail/sendmail.gdf')

2. Create /etc/mail/genericstable file containing local users mapped to desired addresses:

exampleuser exampleuser@exampledomain.tld exampleuser2 exampleuser2@exampledomain.tld

3. Create /etc/mail/sendmail.gdf file containing the fully qualified name of the local mail server.

host.exampledomain.tld

4. Finally, generate a new sendmail.cf file:

cd /etc/mail make all /etc/init.d/sendmail restart DONE ... :)