iPhone accessing Microsoft Server 2003 POP3 Service preventing Outlook access for same mailbox

Question

I'm running the POP3 service on my Server 2003 box, and have just recently run into login issues when accessing the same account with multiple devices. I've been running the server for years, with no problems, getting my mail using Outlook. Recently, I got an iPhone and set it up to check all of my existing mail accounts as well (an Exchange server account, my GMail account, and also my home servers account where I'm running the POP3 service).

Microsoft Outlook on my laptop started failing to log into the mailbox. Restarting the POP3 service and testing it, all was good again. But a bit later, it will start with the login failures again. It seems that as soon as the iPhone checks the mailbox, Outlook is denied access from that point on.

Is this a bug? By design? A problem with the POP3 service? Or a problem with the iPhone not releasing it's connection?

And yes, switching to IMAP would be better (someone is bound to make the comment). For a home server that rarely sees any traffic, it's served my purpose just fine for many years, and if it's as simple as upping some number somewhere in the registry I'd rather not spend the time configuring a new service instead right now.

Thanks.