BleepingComputer.com: OE6 dropping/blocking mail from frequent 'good' senders

So far this issue has eluded the best and brightest folks, but I'm still trying to solve it.
Several people who frequently correspond and collaborate via email have, in the last two weeks, been unable to receive mail from one another using Outlook Express 6. Among these frequent-collaborators is one exception. Her email are received by everyone, and vice-versa. That one user is the only one in the "circle" that is using Windows XP Home Edition SP2. All the others are using Windows XP Professional, SP3, and may be updating their Microsoft yarn ball automatically. I have not looked personally at every system to verify this... yet. All the systems are using AVAST Home Edition [free edition] as their virus AND scanning program. they typically stay behind a router [I've checked it], but several of the systems are laptops and they do take them places.

All the involved/affected email addresses are from the same domain, which is their ISP [Grande Communications]. These are typical Pop3 type accounts with no special securities wrapped around them.
If one uses Webmail via the ISP website to view mail, we see that messages that are missing from the OE6 inbox are there/present on the server, that is, since we set the flags to "leave mail on server". prior to that, the mails would be deleted from the server, irrespective of whether the mail would arrive intact in the OE6 inbox. This, to me, is significant.

I grabbed two newly-minted email addresses from the ISP, and am experimenting to see if I can "keep" them from falling into the same black hole.
Other steps we have tried:
- removing redundant fiirewalls. we're now only counting on the router, but stopping the Win firewall produced no change.
- We tried stopping all the AVAST realtime scanners [although Combofix seems to think there is one or more 'hidden' AVAST process that is invisible to me...]
- I have so far found no notes that might implicate AVAST 5, although that possibility is very much on my mind.
- There are NO OE errors generated to report. amazingly...
- I have just recently turned "On" the OE logging for "mail" and "IMAP". collecting logs now.

The ISP tech guys, at my request, have whitelisted the entire domain for the sake of these users, but so far that has had no effect on the problem.

If anyone has a clue as to what is causing this blockage, please respond!!! Tell me some more steps to take.

Maybe I'm not using good terminology for this issue.

Let me ask it this way, in the affirmative: is there a way to FORCE OE6 to receive/accept mail from certain senders? The broad shotgun of 'whitelisting' at the ISP didn't help.