Exact error message is "Login to server XXX with username YYY failed." It correctly states the server and user name. This issue has recurred intermittently over the last couple of days, several days after the last Thunderbird update. It did not immediately start after the last update. Today it is recurring every time I try to send an email from a certain account. Receiving email is usually not affected. I have no issues with my Yahoo account. The troubled account settings are as pictured below. I have already gone through tech support with Bluehost. They see no errors at their end and they sent me here. But it does seem that if Thunderbird were really causing the problem, by not sending the password correctly, that should happen with all my mail servers, not just one. I already tried deleting both the mailbox and SMTP passwords in the password manager, and then saving them afresh with a freshly set password. No difference. Before this issue, I had not made any changes to my server settings in at least a year. Here are the outgoing server settings: Description: <empty> Server name: box850.bluehost.com Port: 465 Connection security: SSL/TLS Authentication method: Normal password The username is irrelevant and has not been changed. It still matches my domain name at Bluehost. I tried changing the connection security setting to STARTTLS. That automatically changed the port to 587, but it made no difference to the issue. I added Thunderbird to my Windows firewall permissions. It's very strange that it wasn't there already! But that did not resolve the issue either -- and it shouldn't have, since I didn't remove Thunderbird from the list. I had never brought up that dialog before today. A related page here suggested turning off various levels of Antivirus software. I use Avast Security. It doesn't seem to have any setting that would restrict outgoing emails. And if it were restricting something, the Thunderbird error message should be something more like "Unable to reach server", not a login failure message. And it should affect all email accounts that use the same settings. I have another username at the same server and I see that that account is suffering from the same issue. Another related page suggests deleting three files from my default profile, logins.json and a DB file or two. That would seem to get rid of all kinds of stuff that shouldn't be gotten rid of. I hear this kind of advice from bank CSRs, telling me to delete all my cookies. That makes me want to blow my cookies at the CSR! They have no idea how many websites I would have to re-enter my information for if I were to delete all my cookies. When I press Cancel in response to the "Retry / Enter new password / Cancel" message, another error message pops up: "Sending of the message failed. Failed due to unexpected error 80004005. No description is available. The message could not be sent using Outgoing server (SMTP) box850.bluehost.com for an unknown reason. Please verify that your outgoing server (SMTP) settings are correct and try again."