Sounds similar to a problem I had recently click here
Send mail OK
Web mail OK
Two e-mail accouts.
I was using WinME, OE6 and have AVG & Zonealarm installed. ISP is Virgin.
Tried numerous things without solving the problem. Eventually, a Format and re-install cured things (which I am not suggesting that you do, but it showed that the problem was with the pc).
Suggest you try some of the things suggested in the linked thread such as deleting accounts and recreating them.