I really don't think the appearance of the calendar (Lightning) is part of your issue. But its automated inclusion with Thunderbird coincides with several changes to overcome breakages in the security systems, whereby breakable ciphers using 512-bit keys have been replaced by stronger (1024-bit and up) keys.

A look at your error console, which ISTR is located under Tools on the menu, and searching for references to "Diffie Helman" would indicate if cipher strength is indeed part of your current difficulties.

Had to go chasing rabbits over the last few days to figure out why 31.8.0 and 38.1.0 wouldn't work with an old Exchange Server 2003 account via IMAP. Given that there really isn't any way to improve the SSL3 configuration on a production Windows Server 2003 box, my ultimate solution was to set:

security.ssl3.rsa_des_ede3_sha;false

Just a thought, what type of firewall do you use? Could it be the firewall that delays / prevent you sending?

Thank you to all who tried to help. I assumed, as I noted above, that since my inability to send out email messages began at the same time that Thunderbird added a calendar these things were related. Now I think I was wrong to make that assumption. I apologize to anyone I may have misled about this.

What solved my problem was changing some of the setting on the "SMTP Server" page. I wish I could tell you why it works, but I can't. All I can tell you is that I can now send out email messages using Thunderbird.

re : I think I remember that it used to only put one icon there to indicate that I had the Thunderbird program open.

Right click on task bar TB icon and select 'close'. Close both icons. Restart Thunderbird and test send.