I can’t send anything out through any of my Gmail accounts, since yesterday, 04/26/2021…the day I updated to 89.0b1 (64-bit). I can send just fine, using my own server’s smtp, if I switch to one of my own account usernames in order to send.
I’ve checked the setup info on Google, and it hasn’t changed.
I’ve just seen that someone using 89.0b1 (64-bit) on macOS Big Sur is having a similar problem.
I’m RECEIVING my Gmails perfectly…it’s just the SENDING that’s hanging up.
Have you tried sending a whole new message vs doing a reply?
My mac user here discovered that he couldn’t do a reply, but could send a new message. He later discovered he could do a reply, as long as he deleted the text from the message he was replying to.
This needs to be fixed immediately! 
See if the reported problem with the main release, helps to solve your problem.
Does it work if you change the two preferences noted in the pink text here?
https://www.thunderbird.net/en-US/thunderbird/88.0beta/releasenotes/
Is the authentication on the incoming and outgoing gmail servers OAuth2?