Is there a limit to the number of emails or their collective size?

Neglected to add that I'm using filters extensively and have about 50 local mailboxes.

Make sure you don't subscribe to the 'All Mail' folder assuming this is an IMAP account.

You can configure synchronization for individual folders, so you can decide what you download to your local disk. Note, synchronization means downloading an offline copy, it isn't the replacement for a backup.

https://support.mozilla.org/en-US/kb/imap-synchronization

Use the archive feature, so that messages get spread across multiple folders.

http://support.mozillamessaging.com/en-US/kb/archived-messages

Create a backup of your profile on a regular basis.

http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

That was most insightful, but I have been unable to apply it. I did go in to the Synchronization & Storage - Synchronization - Advanced and UNCHECKED both All and Important. I was able to satisfy myself that if I deleted the "Important" folder on my PC that I would not lose the message where it had been re-directed by a filter. But the All folder continues to grow. I have restarted Thunderbird several times, and I have rebooted the entire computer several times. When I was accessing my gmail account via web mail, I saw a message that says ... "''Moving conversations from All Mail to the Trash will remove them from all views, including labels and search results. This is because in Gmail, a conversation can be displayed in a variety of views, but there is only one copy. Are you sure you want to move the only copy to the Trash?''" That may not apply in Thunderbird here on the PC, but it makes me leery of simply deleting the All folder on the PC. I think I would be more comfortable if we could find a way to stop the "All" folder from growing. It's now up to 115,786 messages taking up 1.746 GB, and both are still growing.

Right click the account in the folder pane select subscribe and unsubscribe from the folder.

Simply viewing the content initiates a sync, unsubscribe means you don't see the folder.

I am excited with all this new knowledge and simultaneously humbled by what I don't know. Interestingly, Matt's instruction seems to work even though I had previously done what appears to have been the same thing (obviously it wasn't) by selecting the IMAP account, selecting View Settings for this Account, then Synchronization & Storage, then Advanced, and de-selecting the All and the Important boxes.

Overall, it seems to be a bit faster; but at times it's still slow with an occasional "not responding," but no crashes. I'm working to move older messages out of frequently used folders so as to keep them under 1000 if I can.

My understanding is obviously weak in several areas. One has to do with multiple folders. In that left pane, under the primary account which is IMAP, there are, among other things, a "Sent" folder and a "Gmail" directory. In that "Gmail" directory, there is a "Sent Mail" folder. I have just observed two things. One is that when I send an email, a copy appears in each of those two places here on the PC as well as in the "Sent Mail" folder on web mail. The other is that there is a difference in the message count between the "Sent Mail" folder in the Gmail directory here on the PC (5,750) and the "Sent Mail" folder on the web mail (4,470). Why aren't those two numbers the same?

Back to the PC. in that Gmail directory, there is also a duplicative "Drafts" folder; and there is a "Spam" folder with the same icon as a "Junk" folder.

With thanks to both of you, I'm better off than I was. I want and need to know more about IMAP. My 1200-baud MODEM communications go back to the early 1980s; I feel like an old dog trying to learn some new tricks. Can you add to my education directly or point me to where I can get some more knowledge?

Thanks.

There is a difference.

The Advanced button in Synchronization and storage sets folders to be synchronized.

Right clicking the account selects a dialog that sets folders that are "subscribed" to appear in Thunderbird. It is a subtle difference and one it took me quite a while to find.

You can also set some of the synchronization defaults on a per folder basis by right clicking the folder, selecting properties and the synchronization tab.

Thunderbird maintains one folder structure regardless. "Local Folders" This is a special account and stored all mail locally. Make a good place to backup mail to if your quota is disappearing fast (not usually an issue with gmail) but it provides local storage, and is a great "global inbox" it's original job, for multiple POP mail accounts.

All IMAP accounts appear in the left pane as the synchronization means all their folders have to be maintained separately, otherwise it is just to complex. So in that case you get two send folders two trash etc.

To get the best "grasp" of the folder structure is to ensure folder on the view menu (Alt+V) is set to all. Change it latter by all means, but all makes all folders hang of their real parent, not a virtual one.

In relation to gmail and outlook.com. Both add your send mail from the SMTP server to your sent folder. This is not within the standard, but they do it anyway. Result is two of everything in the sent folder. The option there is to have two, turn off Thunderbird storage of the send, or place the initial one in local folder sent and allow gmail to do it's thing.

Location of the "sent" can be set of the option disabled in account settings under Copies and folders

A reasonable starting point for information on IMAP in general is Wikipedia. There article is a good grounding and general history. see http://en.wikipedia.org/wiki/Imap

It also links to the relevant RFCs. Heavy going usually, but sometime necessary.

Slowness in Thunderbird is often attributable to anti virus actively. Thunderbird stores mail in quite large files (sometimes 4gb) each time these files are opened, some AV software wants to do an on access scan, which takes inordinately long because the file is large.

Excluding the Thunderbird profile folder from "on access scanning" through preferences in the AV software (the same as is done for exchange for much the same reason) often sees a complete disappearance of slow then fast action.

Note that most limits you see including folder size do not apply to IMAP. The all mail folder saw to that, something had to be done.

it is also important to compact folders. The MBOX files Thunderbird uses and almost the same sort of thing Dbase used in the day, primary data file (in text) and a separate index (MSF) Also like debase deleted mail is marked as deleted and not displayed but not removed until you compact. So deleting and moving actually increases storage requirements until you compact.

It also allows add-ons like this http://en.wikipedia.org/wiki/Imap to undelete mail until such time as the folder is compacted. You will not an expunge on exit option on the server setting. That is basically a compact, and it sees quota released on the server.