are you looking inside Thunderbird or in your file system for the archive? It's location is derived from the setting in Tools menu (Alt+T) > account settings > copies and folders.

While in account settings check the disk space entry and make sure you have not set it to delete messages over 60 days old.

I am thinking about Archiving as the same thing as an Outlook Archive, meaning - a completely separate set of folders mirroring what is on the server. That isn't the case, is it?

And <major expletive>, you were right. The "Delete all mail older than 60 days" was set. In my wildest imagination, I cannot imagine ever setting that. I would not willingly delete my wife's message store of 14 years.

<Another major expletive>! Glad I air conditioned the dog house in June. Gotta start thinking about getting a heater. She may let him in, but this is gonna be home for a while.

Lou

Now we have a cause, try this add-on on the folders in question. If they have not been compacted you may not need that air conditioning

https://nic-nac-project.org/~kaosmos/index-en.html#recDelMsg

And Nope Thunderbirds view on archiving is not the same as Outlooks, sorry to say. Personally I made my own,

It is a bit tedious, but if your interested in setting up a dummy account to do archiving duties I will explain what I did. Moving Archives off the machine to a NAS was the original goal at it worked.