What is your anti-virus software?

I am using AVG (which I keep updated regularly). I temporarily disabled it, closed and re-opened Thunderbird and tried to send an email. Same problem appeared.

Try to start *Windows* in safe mode with networking enabled.

- Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

- Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Oddly, today this anomaly is not happening. Last night I did some cache cleaning, updated T'bird to the current version, and restarted. So I don't know if any of these steps are responsible, or something else spontaneously corrected the problem, but now my replies are being permitted to go out.

I won't close out this thread in case the problem recurs. Thank you for your suggestions. I will hold the last one in reserve in case the problem reappears.