Error sending messages in Thunderbird

The error is that Thunderbird refuses to send mail over SMTP, even though it can receive mail.

When I tried to send email, Thunderbird spewed forth this error message:

Sending of message failed.
The message could not be sent because connecting to SMTP server mail.mitre.org failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.

After a few days of experimenting, checking every single setting in my Thunderbird profile, swearing mighty oaths, and cursing at my need to resort to using webmail, I finally found the answer.

I rebooted.

The problem went away.

I have no idea why this has happened, twice. The first time I encountered the problem I ended up completely deleting my profile and reinstalling Thunderbird before I found the answer. I've only seen this problem on my Windows XP laptop at work, so it may be a problem with the Windows version of Thunderbird.