Since the recent update of my Thunderbird Beta, I have been able to receive all of my email without difficulty. However, I have been unable to send via Thunderbird. To send emails, I must go to my Earthlink webmail account or use my cellphone where I can send without difficulty. 
Thank you,
Karlynn Morgan
Karlynn, I had exactly the same problem and also use earthlink. Be careful if you try to revert to the earlier version or go to the regular channel, as you may lose your profile. That happened to me and I had to go to System Restore, to restore to just before the TBird Update, which worked thankfully. I have tried to find out why this happened but run into blank walls and empty chats on it, until I found yours.
Just wanted you to know you are not alone! In the meantime, I have TB-beta 87.b3 back up and running correctly and have disabled automatic download/installs until a fix notice is made.
~ John Kirkpatrick
Thanks John, Iâll just stick with webmail for now and keep checking for a fix. The last time I tried System Restore, I lost all of the email in my saved folders which was quite a painful experience for some time.
Karlynn
I have the same problem with two different ISPsâŚunable to get email. I tried downgrading TB and then I went back to the beta version. Perhaps just waiting till the next upgrade, as you suggested, is best.
I am also experiencing the same problem (Ver 88.0b2). I can receive email and update folders and calendar, but cannot send. My ISP is âSHAW.CAâ
I can still send using smtp.mail.yahoo.com. This is just affecting my SHAW accounts.
I even went into âconf editorâ and tried changing the âsecurity.tls.version.minâ value to 1. This didnât make any difference.
Try searching or asking on Thunderbird Support.
You could also try Help > Troubleshoot Mode, or create a test profile, then create and test the account.
I tried searching Thunderbird Support first. I have also tried âTroubleshoot Modeâ.
I also tried creating a test account. And I even deleted all my saved passwords and reentered themâŚ
Iâm using macOS version 10.15.7 and Thunderbird version 88.02b. This problem started with the update to 88.
Just to sum up: " I can receive email and update folders and calendar, but cannot send. My ISP is âSHAW.CAâ
I can still send using smtp.mail.yahoo.com. This is just affecting my SHAW accounts."
Question: Is anyone looking into this problem?
The security for mail.shaw.ca on 587 is STARTTLS. If it still wonât send with 88beta, revert to the old sending process by changing two preferences, as explained in the release notes.
https://www.thunderbird.net/en-US/thunderbird/88.0beta/releasenotes/
This âfixâ did not work for me. Any other possible help?
Awesome. Thanks for the help. This was the fix.
Iâm actually embarrassed that I missed this bit of info in the release notes. That will teach me to read and ârereadâ this info.
( preferences âmailnews.send.jsmoduleâ and âmailnews.smtp.jsmoduleâ to false.)
Jeff
Overnight this âfixâ worked. Iâm glad to be back to using Thunderbird again. Webmail is so tedious. Thank you!