What's the best / easiest way to combine two mailboxes on Exchange 2007?

Solution 1:

You can use the export-mailbox cmdlet to export your mailbox to a .PST file, and then the import-mailbox one to import the data in the target mailbox.

You can also use export-mailbox to directly move data from one mailbox to another, but then you'll need to save them in a subfolder of the target mailbox.