Can no longer send new emails or forward existing emails

I'm not sure exactly when this problem started but it could have been when I '''had '''to move my primary email from Xfinity over to Yahoo. Also not sure if this started after a recent Thunderbird update. I can not send new emails nor can I forward old emails. Haven't tried reply but suspect it is the same. This is the message I get: Sending of the message failed. An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again. All settings and passwords are correct as I can receive and send from all my different accounts using browser. I can send emails from my accounts, on my iphone, though not using Thunderbird on iphone. Account settings and passwords are the same on the phone.