I suffered similar problems, which were overcome by accessing my mail via my laptop rather than PC. I don�t know why this worked, but it did � once the laptop had accessed my mail, my PC could once again access the mail as normal.
Then, both failed (although I still had internet access), I telephoned my IP provider�s technical department (in India), who advised that the �fault� was due to my Outlook Express inbox folder being full (too large), and advised not to use Outlook Express. I can�t recall the mail system I was directed to use � but it worked.
Even though my laptop had very little mail in the Outlook Express folders, I deleted all mail in my in and sent folders (on my laptop & PC), which worked (allowing me to use Outlook Express).
But I don�t know why Outlook Express should care how much mail I have saved on my computer � but the man in India assured me that this was the problem (which I did not believe, but it appears that he is correct).