Outlook 2003 addin; items in non-default mailbox not MailItems

Hello,
I am writing an addin for Outlook 2003 that need access to a mailbox that is not the default mailbox. When I select one of the emails in the secondary mailbox and access it like this ActiveExplorer().Selection[1], the resulting object is no Outlook.MailItem and therefore any attempt to cast to this type fails. What else could it possibly be? The same code is working perfectly for items I select in my own default mailbox.
Would be cool if someone could shed some light on this.