Avast 5 is blocking one, but not all, of the mail accounts that I access at the same web host with Vista-based Windows Mail - despite identical account properties in the Tools/Accounts dropdown. (I get the error message ending 0x800CCC90.) This does not happen when I temporarily disable Avast. Assuming this problem has no solution, how can I exclude that particular POP3 account? - there is no exclusion feature in the Mail Shield settings, as there is for urls.
You mean how to exclude server address from scanning?
Try Settings → Troubleshooting → Redirect Settings and “Ignored addresses” in MAIL tab.