[tlug] temporarily blocked by smtp - probably Thunderbird issue

["Micheal E Cooper" <mcooper@example.com>]

I was doing IMAPS with Thunderbird successfully for weeks. Then one day, I
clicked a mailto: link on a web page and tried to send email from
Thunderbird. Thunderbird was not running at the time, but the compose
email box appeared, so I wrote a message and tried to send.

I was answered with an error message from Thunderbird saying that the SMTP
server had refused the connection.
I started up Thunderbird and tried a test, but I got the same error message.
I restarted sendmail process, but no change.
I ssh'd into a remote server and telnetted to port 25 and sent mail using
the same SMTP server by typing in the commands directly, starting with
HELO. No problem.
I sent mail from the webmail interface of this same server. No problem.
I checked with other users of this server. No problem.
It was only my IP that was being blocked.

After three or four days, I was able to use IMAPS from Thunderbird again.
I did not record the IP at the time I was blocked, so I don't know if my
IP address changed in that time or the server just stopped blocking it.

Now, I assume that I was temporarily blocked because Thunderbird tried to
send the message without first properly connecting to the server.

How can I prevent this from happening to other users? I was hoping to move
all users to IMAP{S}, and I would rather not disable any kind of automatic
spammer-blocking on the server. Other than this little hiccup, I have been
ecstatic with Thunderbird as an IMAP client.

I googled for phrases like "temporarily blocked smtp" and "sendmail ip
blocking," but the only hits are about only mail server installation and
configuration.
I have read over the sendmail.cf options and Thunderbird settings, but
nothing seems to be relevant.
The server is out-of-the-box vanilla RHEL3 with sendmail, procmail,
squirrelmail, spamassassin.
The client is running the latest version of Thunderbird.

Any ideas would be appreciated,
Micheal