The calendar tends to have issues with repeating meetings, and possibly with other types of meetings. I've got it disabled on mine at the moment - every so often I set it to do a one-off synch to a new calendar, then delete the old one.

Not sure about sent mails - I can't recall whether I've ever sent anything from the N900. It could be related to the calendar synch issue though - try disabling the calendar synch and see whether that helps.