Thunderbird receives, wont send?

Is anyone here having problems sending emails using Thunderbird?

I have been using 1.5.10 with no problems up until about a week and a half ago, when suddenly it started to have difficulty sending emails, needing several retries before successfully sending.
Now it will not send at all, but will still receive just fine.

I have checked all the obvious things like SMTP settings, password caps lock etc., tried turning off anti-virus and firewall, but nothing makes a difference.

I don't seem to be the only one with this problem either:

http://forums.mozillazine.org/viewtopic.php?t=546933

http://forums.mozillazine.org/viewtopic.php?t=547156

Maybe a Windows update has borked something?

Continues to work fine for me under Windows XP Pro with all the standard updates applied.

What are your SMTP settings? SSL? TLS? Password? Port?

The first thing I'd try is connecting to the SMTP server by hand. Telnet (using your favorite telnet application, or, if you don't have one, telnet.exe from the command prompt) to your SMTP server on port 25 ("telnet.exe smtp.example.com 25"). Then try to send an email using this script:

Quote:
220 smtp.example.com status
HELO workstation.example.com
250 smtp.example.com Hello
MAIL FROM:<email@example.com>
250 2.1.0 email@example.com....Sender OK
RCPT TO:<email@example.com>
250 2.1.5 email@example.com
DATA
354 Start mail input; end with <CRLF>.<CRLF>
From: <email@example.com>
To: <email@example.com>
Subject: testing email server

this is the body of the test
.
250 2.6.0 <blahblah> Queued mail for delivery
QUIT
221 2.0.0 smtp.example.com Service closing transmission channel

Red text indicates server responses, which will be similar to, but not exactly what I listed. Obviously, change out hostnames and email addresses for accurate data.