I believe the answer is yes, it seems to be a server-side IMAP feature.

I'm not quite in your situation. What I do is using two/several IMAP accounts (which is possible in my case) and subscribe to all folders of all (both) accounts on all (both) machines. I do have special local TB-filters on one account on one machine. In my case it doesn't matter when the machine is not on and hasn't filtered yet.

I keep "Message Synchronising" un-checked on one machine, which seems to download the message body only when I click on the message in the message list.

If I understand correctly you have set up filters on your server, not in TB. If you want to keep them (or do the filtering in TB, same filters everywhere) I guess you could turn off the mentioned synchronization, i. e. "Keep messages for this account on this computer" on both machines, which should reduce download traffic.

Not really a good answer. But other than that I'm not sure if IMAP provides a really good solution in your case.

The problem using TB filtering that all the messages could be downloaded to both machines.One local machine would have filters for some messages but until that machine downloads them & filters them they will be downloaded on the other machine if it were on. Sure I could just down load the headers and wait each time I want to read a message on that machine. I could also download everything and just make some folders "invisible" but that sort of thing defeats the purpose & usefulness of IMAP.

I came across a forum where someone was having problems with TB & Microsoft Server. That problem was that TB would "self subscribe" to folders on the server, even ones that had been deleted on the server end, then throw an error message when TB tried to operate on the deleted folder. No other email program would behave like this. This suggest that some sort of "subscription list" for TB was being kept on the Server and kept "synced" with TB, almost like being (subscribed to the list).

There seems little information out there on how subscription physically works (the nuts & bolts) Most of it seems to be "how to configure xxxx for IMAP" .

I guess Its hard to know what to even search for because TB has become so big & cumbersome that only the programmers know what/how various bits works and since they already know, it doesn't occur to them that users may not know.

You are probably right about documentation. There are also some older knowledge base articles: http://kb.mozillazine.org/Category:IMAP

It seems that local filtering in TB would not be appropriate in your case. But perhaps the way you have it now (server side?) is the best you can get?

I'm not an expert, but to me the subscription thing seems to be an IMAP specification and is saved on the server, and perhaps ideally also somehow in the client, when I read this, for example:

6.3.9. LSUB Command 6.3.6. SUBSCRIBE Command 6.3.7. UNSUBSCRIBE Command in http://www.ietf.org/rfc/rfc3501.txt

Also interesting: http://www.imapwiki.org/ClientImplementation/MailboxList#Subscriptions

But note the special meaning of mailboxes, i. e. what the user would call folders.

Probably even the best and strictest protocol implementation would not resolve the probelm of having several clients wanting to have different subscribtions.

Would IMAP4 ACL Extension be a solution, provided your server has installed or supports it? (from http://www.imapwiki.org/Specs, http://www.ietf.org/rfc/rfc4314.txt)

Thanks for the links. This is the 2nd time I tried to post a reply so if the other one pops up just ignore it.

I have seen all but the last link. It seemed to me that LSUB command gives the list of subscribed mail boxes (folders) which is a subset of what the LIST command supplies.

I assumed that the LSUB command would store its results at the server end, but i guess the folder on the server only has to have a specific attribute to keep track of whether its subscribed or not. The Server is probably not "aware" of the client host identity, only that it has been authenticated. Under those circumstances, it could not store client specific subscribe (LSUB) information.

In that case its probable the server just checks the folder attributes when it gets a LSUB command.

I have to wonder if TB has a script to subscibe/unsubscribe at each time it starts. This would do the same thing but only provided another TB (or other email client) was not trying to acess the account at the same time.

I didn't really want to get into the internals of TB all that much.

I could just use local filters to move messages to another account and further filter that account on the other machine.

Its a pity the filters on TB don't have a "In This Folder" or be able to be specific to one folder rather than one account.

Perhaps an alternative to a script might be

https://addons.mozilla.org/thunderbird/addon/imap-acl-extension/

I haven't tried it. But if it works in your case, this might be another approach.