I'm having the same problem starting Jan 10th. I've had an old @MSN account for over 20 years and I tried to send an email from Thunderbird which would not go through.
I've jumped though all the hoops getting a code from my backup account, then being prompted to get a text. I got into my account on line, changed my PW in case I'd been hacked. Back at Thunderbird I changed the PW and was blocked again. I then I ran into the "usage exceeded" problem with my phone. I was going around in circles.
I even filed a report last Wed to have my account unblocked. MS gave me a speedy reply. And their solutions didn't work. So I tried using my partner's phone. That got me in. So back at Thunderbird I wrote my friends I had my account blocked and... once again, the letter would not go out. I was blocked again... and now my partner's number also got the "usage exceeded" error.
I've looked at the responses here. I don't have much mail, I have nothing in my sent box because I move all sent to the inbox. I'm at a loss. I NEED this account back. It's where I do all my business, credit cards, Amazon, health/home/auto insurance, IRS, you name it.
UPDATE UPDATE: I was able to get into my web-based email account... but I could not get a text message on my phone because of that "quota exceeded" bug... but I did get through on my partner's phone. I was able to send a letter from that account while on line. I don't dare to try to send it from Thunderbird again because that's how my account was locked twice.
I await some solution before I use Thunderbird to send anything again and will just use the web-based account