Hi Kathleen,
I have had exactly the same problem for well over a year - it occurs
intermittently for a few days and then clears up.
Messages from certain senders (which are trusted) are immediately deleted
upon download: I hear the new mail alert; can see the header in the inbox for
a fraction of a second, then it disappears, and cannot be seen in junk or
deleted.
I cannot see any common pattern in either the senders or the messages that
are deleted. Others get through OK.
And the senders that are involved change from occurrence to occurrence, too.
This happens even with all junk filtering turned off. I have all the
recommended updates, and no rules turned on that involve deletion of messages.
Tearing my hair out over this as I can’t find anything on MSKB to help.
Hint for the OP: I have worked around this by saving messages on the mail
server for three days so I can use an online reader to check when I notice
that expected messages are not appearing. Without that, I would be completely
stuck! It is hardly an elegant solution though, and impossible to keep
properly in synch with Outlook, as forwarded copies from the web interface
are also deleted.