FYI
The following information is the procedure for solving the pop mail server problem that caused by importing yahoo mail into Thunderbird. In oder to make emails accessible both online in the Yahoo server and Thunderbird, Please see the following:
1. First, to keep emails synchronized on yahoo mail and TBird and make the online server able to receive emails again (instead of sending it all to only Thunderbird), in TBird, Under "tools"--Account Settings---server settings---check "leave messages on server" AND "until I delete them". This makes emails available to both online Yahoo server And Thunderbird. It's a thing of beauty. (now the POP function like IMAP??)
2. Then, download the Fast Mail Redirect add on from Mozilla
https://addons.mozilla.org/en-US/thunderbird/addon/1962
and installed it into Thunderbird (tools->add-ons->install button-->locate the downloaded file on disk)
3. Restart Thunderbird
4. after restarting Thunderbird, under "message" there will be a new option "redirect"
5. In Thunderbird, highlight multiple messages and go to "Message-->redirect" and send it back to original email address (same as the one thunderbird shares). (instead of one at the time--!!!!) go to Yahoo mail, and there's the messages sent back to Yahoo inbox,
6.In Yahoo inbox, select all messages--->mark--->mark as read.
7.After MailRedirect, there will be second copies of emails sent back to into the TBird inbox. In TBird, do "Get Mail" and all new redirected mail will be shown in highlight, hit "delete" and then the copies will be deleted.
conclusion: problem solved! don't take impossible for an answer. !!!