I am assuming that you are using Active Directory, and Exchange 2000, maybe as part of the Small Business Server?

It depends on what you mean. If you want users to be authenticated with domain1 and domain2 for network access(by a single domain controller) and have different email addresses on the exchange server it is not possible because you cannot(as far as I know) host the active directory for 2 different "domains" on the same domain controller.

Having said that, it is entirely possible to host multiple email domains on a single exchange server. you can host both example.com and example2.com on the same server, it just requires some changes(IIRC) to(or add a new one maybe) the default recipient policy, and add the different email address to the user using the exchange properties of "Active Directory Users and Computers". Exchange 2000 as part of small business server may not allow you to do this, I do not know, but I do know there are some restrictions on what it is capable of.

The problem with what you want to do, is really hosting multiple active directory domains on the same server. As far as I know, it is not possible to host domain1 and domain2 on the same domain controller. You may be able to run a second domain controller, which hosts domain2 as a part of the same forest which includes domain1, and set up the users on domain2 on exchange although I have never tried it, and suspect it would not be trivial if it is indeed possible.

If you are willing to have users which would be in domain2, authenticate to domain1 instead, they certainly can be a part of the exchange server, with different email addresses than the users which would be in domain1.

Good Luck,

IchNiSan