Different Emails in Outlook and Mail

Hi,

I'm running Outlook for Mac 2011 with a number of IMAP accounts.

Today I opened Mail and noticed emails in the Mail InBox that aren't in Outlook's InBox (or the InBox for the account or any accounts.) They are all from one sender (from whom I regularly receive emails) during the month of February (though I suspect this last point is irrelevant.)

I've changed my View settings in Outlook and even restored the default view settings.

Has anyone experienced this issue?

Thanks,
Minding