First, 12MB is not an unreasonable size for an empty mailbox in Notes, depending on the template selected. So, that doesn’t really tell you anything. To see if the mail file has any mail in it, bring up the properties for the mail file (Alt-Enter in Notes), go to the second tab, and check how many documents it says are in there.
If there are zero documents, try sending a piece of mail to the mailbox. If there are still no documents, my next question would be “did you create a mail-in database document in the address book?”