I sure could use some help brainstorming this situation which has only developed sometime in the past 15 hours.
I use Thunderbird 8.0 as my email client with my Comcast account. I successfully sent an email yesterday afternoon, but, starting early this morning, when I try to send an email, I consistently get "Sending of message failed. The message could not be sent because the connection to SMTP server smtp.comcast.net timed out." I can receive email with no problem, and my first thought was that the problem lay with Comcast. After a while I tried logging on to the Comcast web site and found I was able to send email from there with no problem. I then called Comcast, and they indicated it must be a Thunderbird problem. They provided me with a tech help number for TB, which I used. The hard-to-understand tech responder logged into my system, but he was only able to confirm that I couldn't send email from TB, and that I could do so from the Comcast site. Further help would cost money, which I declined.
More information: (Reboots have been tried with no success.) This same identical problem occurs when I use my laptop where TB 8.0 is also installed. The two computers use different security systems, including different firewalls. I did run Malwarebytes on this desktop computer with negative results. The only common points with the two computers that I can think of are the email address, and the router and modem, but I have made no changes in recent months. The laptop uses a wireless connection, while the desktop is hardwired to the router.
Any ideas and help would be welcome!