Getting really weird now. I tried to send a message to myself in Outlook. Going from the email address provided by my ISP to one of the accounts hosted on my mail server. It's now in my Sent folder, but it never appeared in my Inbox in Outlook.

The message did hit my mail server, though. It's in my Inbox, in webmail for the intended account.

Outlook just doesn't seem to be retrieving any messages from my mail server.