I noticed yesterday that I hadn't gotten any email in certain accounts since 3/8, only because I have been switching over to gmail imap from Yahoo pop3 and still have both, and sometimes read email on a tablet. The tablet had email that Thunderbird on my main computer didn't.
I don't remember when I upgraded to 8, but it was shortly after it popped up. At some point it said to enable the SSL in the email client, which I did, instead of how it used to be with the Avast setting (no SSL in the client). When I checked the settings yesterday, I looked at the Mail Shield, which had a list of all the accounts from the old setup. It said to delete those and let the client do the SSL. I deleted the list.
However, I too continue to get no email on the Yahoo/sbcglobal.net accounts (POP3) unless I DISABLE Mail Shield. Is there another setting that needs to be changed or is this a BUG?