First things first Stop the Router task. Stop SMTP task
Once you have confirmed that there is no access to the mail.box then you can run the normal domino maintenance against it.
Compact -c -i
This should fix any corruption.
The other thing you could do is take the server down….. rename the mail.box…. start the server. This will recreate the MAIL.BOX. Then you could open the old DB and check to see if there is any emails that need to be restored.
It is at this point you can get users to resend them or copy and paste the emails into the new mail.box (warning this will make you the sender of the email . check this is ok before cutting and pasting.
I hop[e this helps.