I followed this article which I found online to delete mailboxes from Exchange 5.5
To remove migrated mailboxes from Exchange Server 5.5
1. Open Exchange Administrator.
2. In the console tree, double-click the name of the Microsoft Exchange server to which you want to connect.
3. Navigate to the container of the mailbox that you want to delete and double-click it.
4. In the details pane, highlight the mailbox that you want to delete, and then click Delete.
5. When the Are you sure you want to delete this mailbox? message appears, click Yes.
But it didn't work. When I highlight a mailbox in the Exchange Administrator, delete is not avialable. Is something wrong or is this normal. I ended up deleting the mailbox through usrmgr by deleting the user account, along with the mailbox. I'd rather be able to delete the mailbox individually. Any way to do that?