I wrote something daft about message age. Here is a rethink.

Is this account using POP or IMAP? Do you see older messages if you visit the account in its website?

If it's IMAP, go to the Account Settings in Thunderbird and check the age settings under Synchronization & Storage.

Sorry, don't know the difference between IMAP or POP. Setting this stuff up is becoming increasingly so complicated that the average user seldom takes advantage of options. Acronyms abound and I have no idea what most of them mean. Please explain as if I were absolutely new.

POP has been around since 1984, and IMAP since 1988. Neither is a "new" complication.

Google is your friend ("GIYF").

https://en.wikipedia.org/wiki/Post_Office_Protocol https://en.wikipedia.org/wiki/Internet_Message_Access_Protocol#Original_IMAP

Go to Tools|Account Settings|{select account}|Server Settings and it will tell you the answer to this question right there at the top, under "Server Type".

If it is a working account, you may not need to "import" anything. You just add it to Thunderbird and Thunderbird collects what is on the server.

But what Thunderbird finds on the server depends very much on whether this account is set up to use POP or IMAP. Any further and more detailed guidance will depend on the answer to my question about the account type.