is the account configured as IMAP or POP? If POP is the account configured to delete messages from the serv er on download or to leave them there.

Do you use this account on another device like a phone? does it use IMAP or PP these are all relevant questions to try and determine what happens when mail is retreived.

I would point out that I ceased using my Bigpond accounts email many years ago as Telstra demonstrated a long term inability to actually administer an email server, or provide anything that looked like support for their service. Having someone that knows nothing answer the phone is not user support, except at Telstra.

Matt, Thanks for your information. It answered my primary question, and raised a few more. The reason for the differences between the two computers is that the desktop computer is configured for the IMAP mail server while the laptop is set for the POP mail server. It doesn't appear that I can change from IMAP to POP. Also the Mozilla Support material on this subject was last updated in 2018 and the info and graphics don't match up with the appearance of the current version of Thunderbird. I don't know if I can uninstall the IMAP and start afresh with a new setup on the desktop without the risk of losing emails. I'm happy with the setup in the laptop. Regards, Jim Meyland