My hostname keeps reverting to localhost (or localhost.localdomain) - how can I permanently fix this?

This problem is becoming more of an issue because so many mail server anti-SPAM techniques check for proper "HELO" settings from your smtp (mail) server. 

So if your mail server thinks it is "localhost.localdomain", you will find that some of your email gets blocked arbitrarily.  Such is the status of the 'Net these days.

So, how to fix this permanently?  Here are 2 steps that are guaranteed to fix your hostname problem. 

1. Read all the articles about changing your /etc/hosts, /etc/hostname, /etc/resolv.conf, /etc/sysconfig/networks and other files.  Try rebooting, and see if anything works.  Try editing your Plesk configuration.  You might get lucky, and get things fixed that way.  Spend a few hours if you need to - take your time. 
   
2. Now, edit this file: /etc/rc.d/rc.local

Add this line at the bottom:

hostname [your hostname.domainname] (example: hostname mail.abc.com)

Add these lines to restart your mail server (will vary based on your mail server)

qmail stop
qmail start

Try out reboot, and see how it looks. 

NOTE:  This is not the official way to do this.  If you want to follow the official way to do it, see Step 1, and repeat as often as necessary.