Some emails are stuck in the mail.box and if you try to re-save them you got the error message "Can't change document's UNID". The solution to fix that is quite simple.
1.) Stop the router via
tell router quit
2.) stop smtp (if used) via
tell smtp quit
3.) Run a fixup on the mailbox via
lo fixup mail.box
if you have multiple mailboxes you need to run the comand also on the other boxes.
4.) You will then see something like the following on the console:
[11BC:0002-05CC] 2013-02-08 15:15:45 Document NT00D7FF4E in database e:\Lotus\Domino\data\mail2.box is damaged: Document attachment is invalid
[11BC:0002-05CC] 2013-02-08 15:15:45 Document (UNID OFE3206770:32C58AE3-ONC1257B09:00319879) in database e:\Lotus\Domino\data\mail2.box has been deleted