two domains for user logons in AD, one local and one email domain

I have a very simple question but having a hard time finding the answer. When I right click a user in AD and go to properties. I click on the account tab and there is the user logon name. In the field to the right of the name it gives the domain. But there is a drop down list, one is my local-internal domain and the other is our email domain. Not all the users have the same one, they are mixed. What is the difference and which one should I be picking?

Server 2003 SP1 and exchange 2003 SP2
fina27Asked:
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oBdACommented:
That is the UPN, the "User Principal Name"; the normal suffix is the AD domain's name. Most users have trouble enough memorizing their email address, so you can add other UPN suffixes, like your internet domain name (so that users can logon with first.last@company.com instead of username@company.local if you have something like company.local for your AD domain). Additional UPN suffixes are defined in the AD Domains and Trusts MMC; right-click "Active Directory Domains and Trusts" in the left pane, open Properties.
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Windows Server 2003

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