Here's one I just can't figure out. I've got 2 users that get email from an outside vendor: Bob and Bill. When the vendor emails them both, Bill will get the message but Bob won't. I've looked at the headers and the email addresses look correct. The only difference I can see is that Bob is the "to" while Bill is a "CC".
Both Bob and Billl use Outlook. All other mail comes in normally. I've checked Bob's webmail looking for a filter, but see none. The "missing" messages aren't in Junk or Trash...they just don't seem to exist.