There is no option to disable polling of an IMAP or POP mail account. So you can not use downladed IMAP mails as a archive for an account you no longer have as they will be deleted at some point when Thunderbird tries to poll the account, like when you select a folder in the IMAP account to view mail in the folder.

The part about synchronized appears to cause people difficulty understanding it. To archive mails for an imap account they have to be moved from the imap account. Preferably before the account is access is lost as after you have to manually go into the profile folder and copy the files with mail in them out of the profile. Then there is the task of importing them back to local folders using the import export tools. https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/?src=ss

Hi Mat Thanks a lot for this answer! Not what I was hoping to hear, of course. I think this has never happened to me before because for all other cases where I wanted to keep an archive of an account that was going away, I first copied all of the emails to a different gmail account, usually one I created for the purpose of storing archival messages. I think I can upload what I have in tbird for this account to a gmail account? Normally there is more than a tbird store in place, ie the account is still hot. But I can try.