If you get an alert to the effect that Thunderbird cannot connect to the SMTP server, then you might solve the problem with this suggestion.
On Windows XP, open the Firewall settings,
and add Mozilla Thunderbird's executable (thunderbird.exe) to the list of those applications allowed through.
For years this was never a problem for me. In late Spring 2009, there must have been a Windows (more likely) or Thunderbird (less likely) update that created the situation where sending mail sometimes worked fine and sometimes failed.
I use GoDaddy's default port for SMTP, which is 80. Also possible are: 25, 3535 and 587 according to them.
In fact, this didn't work, except episodically right after making the change. Indeed, this is an intermittant problem. Sometimes it works, sometimes it doesn't. I've tried everything from bouncing Thunderbird to bouncing my whole workstation.
My latest attempt was to read "Cannot send mail", which I had read before, but all the way to the bottom. My latest successful attempt is to add port 26, and things started working again. We'll see if that continues to be a solution. I doubt it.
Another good technical note is "http://kb.mozillazine.org/Connection_errors_-_SMTP". This was the note that listed firewall trouble as one potential cause.
Well, that still didn't have any lasting effect. I looked another discussion Postbox in which one respondant said he tried using SSL and it worked. Again, I changed port 25's details to use SSL and it worked (this time). Will it work going forward?