Sometimes I find myself configuring an internal Linux machine to be able to send emails for alerts or from a particular application. Since this will not be a primary mail server, I just want to rewrite the outgoing address to be something that make sense with the proper domain for the users. Here are the quick steps to accomplish this:

 
vi /etc/postfix/main.cf
 

Modify the "mydomain" variable to your email domain

 
mydomain = example.com
 

Make sure to uncomment the proper network interface. I'm usually lazy and just do all.

 
inet_interfaces = all
 

Now at the bottom of this file you need to specify the generic map file for rewriting the address.

 
smtp_generic_maps = hash:/etc/postfix/generic
 

Save and exit main.cf. Now we need to edit /etc/postfix/generic

 
vi /etc/postfix/generic
 

You need to specify the mapping of the original address to the one you want. Since in this case I just want to rewrite everything I usually add the following two lines and it seems to catch everything.

 
 root@example.com   no-reply@example.com
 @example.com       no-reply@example.com
 

Save and exit the file. Now we need to create the postfix db.

 
postmap /etc/postfix/generic
 

This will create the /etc/postfix/generic.db hash file.