The POP3 mail server (inbound.att.net) does not support UIDL or XTND XLST, which are required to implement the ``Leave on Server ... options.

The error message is a bit of a convoluted way to tell you that the server doesn't support that functionality.

What can I do?

Follow the instructions and uncheck 'Leave messages on the server' in your Account Settings.

https://support.mozilla.org/en-US/questions/1157689

I have this problem, three. The answer provided does not help. Additional questions need to be addressed. 1.) How do we get Thunderbird to send and receive email messages to and from a Yahoo! mailbox, using the POP3 service, and still get Yahoo! to leave the original email message on the Yahoo! server? 2.) In the past, we have been able to do this. Why did this suddenly change? 3.) Does this mean that we now have to stop using Yahoo! entirely, and instead start using GoogleMail or one of the other email handlers? 4.) Which other email handlers will allow us to send and receive email messages, using the POP3 email protocol, and still allow us to leave the original email message on the server?

Tax Tom, your question has nothing to do with this thread. Please start your own thread with details of you problem.

FYI: The message from the POP Yahoo server says they do not support Leave on Server when using POP. Thunderbird has nothing to do with Yahoo policies. You can discuss that with them.

You can try to set up a POP account in Yahoo, but as you see it is likely to have problems that are beyond your control. Other providers such as Gmail, AOL, outlook.com and gmx.com provide IMAP and POP access.