If your account is pop, and I assume it is as you tlkingaboutemail not being on the server. There is no connection between the server and the local mail. It is downloaded as stored a a distinct item not something linked to the server at all.

If you have retain on server until I delete set, there will be a popstate.dat file with message ids that is used to delete the appropriate messages on the server. But that will not help you in this quest.

Yes it was done via pop. There were 70K e-mails on the server. All of them got downloaded. Then 10K got deleted from the server. So now there's 60K on the server but still 70K downloaded. I'm trying to find the 10K that got deleted, since I have them all downloaded locally. I just don't know which ones got deleted. That's why I'm trying to find some sort of automation to group the 10K together.

I think the short answer is you are not going to get it. As I said, Thunderbird maintains a file, but if it contains anything is in itself not clear. Have you looked to see if it contains anything?

It may be if it does you could write a script that did amazing things, but it is well beyond the scope of this forum. You would need to employ someone probably. But before you even considered that you would need to know if the file contained some 60 thousand entries. As it is a text file establishing that is not the issue.