There can be several reasons why you did not receive an email:
Occasionally, Outlook goes bananas while updating mails from a mail server, often due to errors in the local copy (called Global cache mode or the OST file).
Follow these steps to rebuild the OST file:
Engang imellem går mail.app i kludder mht. opdatering af mails fra en mail server, ofte er dette pga. fejl i det lokale index.
Den kan genbygges (rebuild) ved at vælge "Rebuild" fra Mailbox menuen
Occasionally, mail.app goes haywire while updating mails from a mail server, often because of errors in the local index.
It can be rebuilt by selecting "Rebuild" from the Mailbox menu.