EternalNewbie said

1. Why is anything archiving itself, if it's only supposed to be possible manually?

GMail has a folder "all mail" which you can see on the web page view. It is what Thunderbird uses as an archive for gmail accounts because that is how Google set up gmail to work. ALL mail is in all mail and copies are displayed in the various folders that gmail created "on the fly" using tags on the mail in "All mail.

2. What determines which accounts will archive messages - in other words, why does only one account have anything in its archive folder?

What determine it is if the server automatically archives. What you appear to have either forgotten or are unaware of is that IMAP mail is synchronized. So you are not only dealing with how Thunderbird works. As an override you have how the server works. Like automated emptying on the trash. Google does that as well. Regardless of your Thunderbird settings.

3. Since three of my accounts created archive folders for themselves, why didn't the fourth?

Archiving has the shortcut A so it is actually quite easy to accidentally archive something and in the process create all the folders to support that. You also do not have issues with automatic archiving with pop accounts, because they are not synchronized.

5. One of those accounts has a maximum message quota. It's set up as an IMAP account, so I periodically clean out the inbox. Is whatever is in the Archive folder for that address still tied to that account, or are Archives local? I want to know if those are counted against my quota.

Change you folder view to ALL folders. IF the folder is hanging off of the account name, then the content is counted against quota. It is the easiest way to describe what belongs to what. You can specify a folder outside of the current account to be used for archiving in account settings. Setting the archive folder (except for gmail) to an archive folder on the Local Folder tree will free quota. Local Folders are local storage on your local hard disk. The only limitation there is the size of the disk and your ability to maintain backup. However using by year folders and "retain original structure" makes finding things easier and keeps the size of individual folders down. While there is no technical limit to a folder size. memory consumption does increase with folder size, so keeping them under say 4Gb per folder is a good idea.

6. Finally - if I don't want messages archived automatically, how can I disable that?

What you can and can not do in a large part has to to with the configuration of the mail server. As I said earlier. It over rides things. Thunderbird for instance can not archive automatically. Servers can and often do. Automated garbage collection and aging of mails are features the server vendors are justifiable quite pleased to offer. What options you get to opt out is also up to the server administrator

. Thanks!

Matt, you are a support GOD. That was fabulous.

Very cool trick with the All Mail view, which was a much faster and cleaner way to understand what's still on my server.

Let me go back to question 4 - are there more emails in the Archive than the inbox because gmail dumps from one to the other? If so, is there some way to find out how frequently it moves them from inbox to archive?

Gmail does not move anything. That is the first thing to get you head around. Thunderbird uses the folder as "archive" so it does not fight gmail in it's implementation.

In GMail everything is in this big bucket they call "all mail". Some of the things in all mail have tags attached to them, like inbox or important or sent.

When you go to the Gmail web interface and click on say sent. Gmail runs an SQL query against the all mail folder and generates what you see as "sent" mail on the fly. Their IMAP implementation is therefore something that is best described as a bastardized IMAP. It works generally as IMAP and the developers of Thunderbird have put in a lot of special gmail handling code. But it is not IMAP. I am not sure of the status of things, but like gmail Thunderbird is no longer carrying duplicate messages for all mail and other folders. But the mechanics are over my head. Suffice to say, unless you want trouble delete nothing from ALL mail. It is Gmails canonical store, everything refers back to that.

To demonstrate what I am trying to say about gmail and it generating folders on the fly.

Press Ctrl+Shift F to open the search options dialog. Search for a regular correspondents email address in your inbox. Once you get some search results, click the save as search folder button. This will save the search in your folder pane as a folder. You click on it in the folder pane and the search populates the folder. This is how gmail populates ALL it's "folders" online. Then they offer some special options on their web site to limit what "Tags" are available for IMAP.

Likewise you were using the unified folder view. The parent folder for inbox drafts etc is a search folder. if you right click the folder and select properties you can actually customize the folders used in the search, so that old folder of business mail "could" be included in the inbox search folders if you wanted. When I check my "inbox" unified folder it is searching 27 folders to build that view of the inbox.

Matt, thank you again for taking so much time!

I'm putting Tbird on hiatus until I can get Theme Font and Size Changer working again. Even if I never use it again, this has been really helpful. Much obliged!