The following codes illustrate how you can setup an alias. This step is optional since we are going to configure virtual mail domains later in this howto. I have added this step to make sure you understand how you can do this in case it is required.
Create a user
sudo useradd -m -s /bin/bash sysadmin sudo passwd sysadmin
Edit the alias table
Open the alias file with:
sudo vi /etc/aliases
Add the following code:
fmaster: sysadmin
To make your changes take effect type:
sudo newaliases
To test your changes send a mail to fmaster and check the mail in /home/sysadmin/Maildir/new folder.
Creating an alias for an account