I have a problem creating the same mailbox names with different domain names at one server (one site). How can I go about doing that? Or can I only assign the name firstname.lastname@example.org once? I would like to use email@example.com, etc. This all needs to happen on the same Web server that I have.
Assuming you already have the appropriate MX records or configuration to get all of the mail from those other domains sent to your Exchange server, adding additional addresses to a mailbox is a simple task.
Go to the properties for that Mailbox in your Exchange Administrator, then to the E-Mail Addresses tab. Click the New button and add another SMTP/Internet address with whatever username.domain name you like.
Note that if you have more than one SMTP address for a mailbox, you can choose which one you want to be the default Reply To address by selecting that address and clicking the "Set as Reply Address" button at the bottom of the dialog box.