Use of UPN Suffix in windows 2003?

Suppose you have a company domain as microsoftsccm.com, as users when they log in they many feel the domain name is too long, so we as administrator can set up Alternative UPN 

(user principle name), upn-suffix which is set:

We need to go to AD domains and trusts àright click on AD domains and trusts node-àproperties -à and add UPN suffix as micro.com, so now users can log with this micro.com also.

Now you need to go to each users or select all bunch of users, go to there properties àaccounts tab and select UPN suffix and select micro.com