a best approach might be to capture the actual error message (the print screen on the keyboard and past into windows pain program, then edit to only the error not the whole window.

Once you have the message attach it to a message reply on the forum using the Add images button under the replies.

Good suggestion. I will do that when/if it occurs again. Thanks

As suggested, the error pop up is included. This message pops up occasionally on launch. I have sent no messages since launch or in a previous session. There are no messages in draft or outbox folders.

Any suggestions would be appreciated.

571 5.7.1 error indicate some "spam" being sent. The user mentioned in that picture sounds familiar to you (is it yours)? Do you see that error without sending mails or when you try to send new mail or to reply to someone? Do you have an antivirus on your computer?

I am the user. Yes, I am running an anti-virus. I also have a professional grade network in my home (like I need another hobby) with full firewall and all my IOT on a separate VLAN.

The error appears upon launching Thunderbird. No emails have been sent. When I actually send emails, they work fine.

I appreciate your question and interest.

Do you have smtp set (with authentication) on Thunderbird with same user/pass as the incoming?

Yes, it is set up for authentication with the same username/password.

So this is an error offered by your mail providers SMTP serer that Thunderbird is simply reporting as why the server refused to send the mail.

TO be in this state there is clearly mail being send trough Thunderbird that the server is refusing to accept for that reason. Unfortunately what the actual content of the mail is may be the issue, things like signatures with social media images on them as by far the most common cause of such issues when you are intentionally sending mail.

The issue here is you say you are not sending mail. This does not compute as there should either be an active compose window or mail in your outbox folder. Did you at some point make changes using the config editor to enable immediate ending of mail when you click send so you can get back to work faster?

I agree that it doesn't compute. No mail is being sent. It occurs on launch. I made no changes using the config editor.

Then we are on the lookout again.

To try and "refine" the issue before you close or leave Thunderbird take the program into offline mode (no internet connection attempted for anything.) by clicking the small ((.)) type icon in the bottom left of the Thunderbird window. The only change you will see is a red line goes across the icon. However if there is mail pending being sent it will pop up a dialog asking to go online to send mail.

Access the options and scroll down to offline. Click the button and change the default setting to startup offline. and make sure ask me is selected for send unsent messages.

Now lets see if it is starting up or going online that causes the issue you are seeing. (clicking the corner icon removed the rad bar and goes online

Thank you for that suggestion. I will give it a try.