email won't send

Is this just when sending email, or when retrieving it, too?

From what I can tell, that error message happens when a user repeatedly tries to engage with the server but is supplying an invalid password, so they lock you out. The block should be dropped after a few hours of inactivity. Have you recently changed your Comcast password? If so, you need to change it within Thunderbird. If you go to the password manager (Tools | Settings | Privacy & Security |Passwords,) you can delete the entries for Comcast and you should be prompted for your password next time Thunderbird engages with the server.

Do you other devices, phones, tablets, computers, etc. that you get email on? You have to change the passwords there as well or they may keep hammering the servers with bad passwords and you will get locked out again.

If, after you have made sure that all passwords are correct, and have let some hours go by for the blockage to automatically lift, and the problem persists, you might have to contact Xfinity for help.

I have tried what you answered, but I still can't send email with Thunderbird. I receive mail easily, so I can't figure out what else might be wrong. Thanks for answering me.

Did you try to remove the Outgoing Server (smtp) and add it back to see.