You aren't telling us much. Did you do a complete fresh install, or did you have a profile from old computer that you used? This is a POP account, I presume? If POP and if you used the old profile, exit Thunderbird, look for a file named popstate.dat in your profile and delete it. That should eliminate the restriction.

Thank you David. I'm afraid I'm not very computer literate but I will give that a try.

If you use windows 10 or 11, rightclick the Windows icon on tray and select 'File Explorer'. Then, scroll to c:\users\<yourID>\appdata\roaming\thunderbird\profiles\<yourprofilenamehere> and look for popstate.dat.

Okay, Linux Mint. If you can see the old messages online, then a fresh install should make them all available to you. I don't know Linux well, but I think the folder is .thunderbird. If there is a file there called popstate.dat, it should be removed. I am assuming you placed the old profile on the new computer.