Upon Send/Receive there are no new messages, although there should be.
Our mail system uses a load balancer that may, when the server is under heavy load, say that there are no new messages when there are. When the load returns to normal, the messages will be available.
Occasionally users may see duplicate (or more) of the same email in their mailbox. This usually only happens during periods of high load on the POP server, and when the customer has the “Leave Mail on the Server” option selected in their client configuration. If you want to leave mail on the server, you should use IMAP, as this is the whole purpose of that protocol.
Outlook cannot download messages/ "1970" emails
Occasionally messages may be delivered to a mailbox with an invalid date, in which case, the mail system tags them with a date of January 1st, 1970 or similar. If this happens, Outlook (in particular) will not download the messages from the server. Using an IMAP client usually allows access to these messages, which can then be deleted.