Source: http://blog.hyperfive.com/2013/07/how-to-setup-internal-smtp-service-for.html
Also
Configuring the server so you can send mail using mail.domain.com instead of ‘localhost’ or the server’s IP address.
If you add the following line to the server’s hosts file it will map mail.domain.com to 127.0.0.1 which is localhost. Please note this would need to be done on each server as reverse NAT translation is not supported.
127.0.0.1 mail.domain.com
How to edit the server’s hosts file:
http://www.howtogeek.com/howto/27350/beginner-geek-how-to-edit-your-hosts-file/