I have a 14 laptop and desktop computers on my LAN, some running WinXP, Win7, Win8.1, and two with Win10beta. About half are usually on, with Thunderbird running on two or more at any time. I have AT&T Yahoo mail service. Thunderbird is my mail client on all, and usually works fine. A few months ago, and a month ago, and again today, I found I had no new messages since early AM. On all computers Thunderbird stopped at the same point, with the same last E-Mail message. "Get Messages" resulted in about 2 minutes of churning away, followed by "No New Messages". A repeat of Get Messages" is sometimes the same result, or sometimes a message that the mail server did not respond, or password not recognized. A reboot does not help. Same on any of the computers. I found that if I go online to my mail server web mailbox page, and delete the last message Thunderbird was able to download, the problem is cleared. I can now download all the messages as usual. The message apparently causing the jam was in all cases an advertisement (but not the same sender), with lots of external graphics content.