You are not crazy about this!
Please excuse my jumping into your thread, I have had the same problem since I installed ver .518 of Avast the other day. When I was looking to this forum for help I saw this post and also that another newer Avast was avlailable. Like you, I saw it work and then today my wife calls me at work saying it doesn't. That kind of troubleshooting by phone I don't need!
The new one, ver 4.5.523 seems to have fixed the problem for me.
This is what I did:
I have a very similar software setup to yours, XP with SP2 patches, XP firewall off, ZA ver 5.0.590 with ashMaiSv.exe given full permissions. OE express didn't connect to download mail. After timming out, an Avast message poped up saying the server yada.yada.yada.yada port 25 could not be reached. I pinged it and it is there.
Turned off the internet mail provider and low and behold all the messages come in! Turned off ZA (not recommended, but I am behind a router) and repeated the same thing, Avast Internet provider on == no messages. Avast provider off OE works again.
So it WAS Avast blocking the messages.
Next I removed ashMaiSv from ZA program control to allow ZA to redetect the program with no change after giving permissions.
Then I enabled Avast's program upgrade and it now works with the Internet Provider running.
So that is it, at least right now, ver .523 is working. We'll see if it is intermitent, but I would think not.
May I suggest you uninstall Avast and reinstall it with the current version? Perhaps you had a bad setup....