You should talk to your email provider. The error is from their server.

You may need to configure different SMTP server addresses corresponding to your different email accounts. In fact I think you normally need to do that so Thunderbird authenticates with your credentials for the appropriate account.

Okay I have a similar issue. See attached screen capture. My ISP is AT&T using Yahoo mail servers. I have a primary email account and a sub-account. I started experiencing issues with sending out messages from the sub-account a couple of days ago after the upgrade to the current version of Thunderbird. As a result of your reply above about multiple server addresses I added an additional outbound server with the email address for the sub-account. I set that outgoing server as the default. Now when I attempt to send out a message for the primary email address I receive the same error that I had for the sub-account. But now that the sub-account outbound server is set as the default I cannot send messages form the primary account. Extremely frustrating to say the least. AT&T support cannot provide any explanations as they want everyone to use web mail which I prefer not to use. Any suggestions on why this occurring and how to resolve? I receive messages in any of the account but its the sending that cause the issues. Thanks.

I don't have Thunderbird on this computer, but hopefully you can find a way to pair the correct outgoing server with the applicable incoming server account. It is impractical to switch back and forth...

Yes I agree that this switching back and forth is getting tiresome. That seems to be the only way to send these messages. Setting one of the AT&T outgoing servers as the default, which is required, seems to be the issue as the gmail accounts have there separate outgoing server email addresses and I have no issues with them

I figured it out. In the Settings for each email account, at the bottom of the screen is a drop-down box to select the outgoing SMTP server. So by setting up a separate server account by email address and then selecting the appropriate outgoing server tied to the appropriate email address, the sending of email messages is again working properly. This appears to be an AT&T/Yahoo configuration change that occurred this week and as usual their support people know nothing about it. Thanks.

That didn't work for me. If I set up a default SMTP for each account, the default reverts back to the last one I set up. I went back and double-checked. For example: I have a GoDaddy server, smtpout.secureserver.net. I also have a Yahoo and an Ameritech email, both of which use ATT's SMTP. My gmail server is smtp.gmail.com. If I default the ATT SMTP, it only works on the Ameritech and Yahoo emails. It won't work on the GoDaddy email. The only answer I can see for this is to allow each email to configure it's own outgoing SMTP. Can anyone suggest a solution? The strange thing about this is that the ATT default doesn't seem to stop the gmail email from sending. The only one I'm having problems getting to work is the secureserver SMTP.