I have had exactly the same problem since 11th Jan. I can't send and got locked out of my account. I reset password to recover the account but still could not get logged in to my email ID as verification said too many attempts try tomorrow.
Reading the comments I realised I was using the same phone all the time for a code.
I used a different phone, got a code code and got into my email for the first time in over a week. Thank you so much for the advice.
I also have huge old sent file so deleted a lot of that as soon as I got in.
I also use thunderbird.
I haven't dared to send anything yet as I don't want to get locked out again. I will wait a few more days
As someone said it is hugely stressful as so much is linked. At least I can see important information again.
Microsoft had replied to my customer issue ticket ON MY LOCKED ACCOUNT!! despite I asked them to use a different one. They didn't tell me what to do.
If Microsoft know about this problem why are they not offering advice and an estimate for resolution.