Continual message send errors

[BillC]

I continually experience errors sending mail from Thunderbird, using
Yahoo mail and the Yahoo and Webmail add-ons.  This has been a
continual problem through multiple updates of both Thunderbird and the
extensions.  Currently I am on Thunderbird 12.0, Webmail 1.4.7 and
Webmail - yahoo 1.5.7. Oftentimes, the only way I can send email at
all is to go to the browser and use the Yahoo interface.  Do any other
extension users experience this?  What is going on and what can I do?