message "Login to server smtp.mailbox.org with username myname@mailbox.org failed

Why am I'm getting message "Login to server smtp.mailbox.org with username myname@mailbox.org failed” when sending message in Thunderbird. I have checked I have all the right settings for incoming and out going servers ect... I don't have 2 factor ID set up. It happens for both my main/real email as well as alias emails. Also I am getting constant popups asking for password. which I enter and they keep coming.