Um, there isn't a simple answer. Users occasionally ask for this, but not often enough to drive any change.

The challenge is that the data is split over multiple files. It would be quite a complex task to extract the required data relating to the original account and then insert that data back into these files in the guise of a new account.

I'd be inclined to set up a separate new account and leave the old one alone. You say you archive to Local Folders; do you habitually route all messages that you want to keep into Local Folders?

Did the old account use POP or IMAP?

If one is POP and the other IMAP then it would be unsafe just to change server settings.

It is possible to move a filters file form one account to another, but that doesn't answer the problem of re-creating the original folder structure in the new account.

If the old account is POP and the alternative account can be set up to use POP, I'd suggest you set up the new one as a separate account. When (if) it all works, then you could transfer the new account's settings to the old one.

Personally I have eight or so accounts. Some users have dozens. So trying to imagine wanting to limit it to one account is bit of a challenge. ;-)

How long will the mail addressed to the old account continue to be forwarded to a new account? What promises have been made? Do you have this in writing? If the original account's provider is going out of business, (you did say "shutting down") I wouldn't expect it to continue operating for too long.

Alexander again... Another approach might be to apply filters to the unified inbox instead of just to the account inbox, but I haven't found a way to do that. It's looking like I have to manually re-create all 34 filters presently used on my old account into a separate identical list for my new account before I discontinue the old account. is that really the case?

When I say "Can I just enter the new address (and its server linkages) into the old-account info in TB? I will no longer need any of the old-account structures" above, I am talking about the "Server Settings" in the View Settings for this Account" pop-down. Were I to essentially rename the account and change its server settings, would all the filters still apply and send messages to the same Local Folders?

Thanks again (sorry I wasn't as clear as I would prefer)...

OK, Zenos, thanks. The old account is POP and the new one is IMAP, so I won't try to "convert" the old account into the new one. The IMAP new account is presently set up and appears to be working, but the folder structure presented me as native to that account does not include Local Folders.

I do re-route all the messages I want to keep into the Local Folders structure. Is the Local Folders structure available to only the original account or can it also be used as a filter-destination by filters set up in a new account?

Is there some way I can duplicate all the filters set up in the old account into the new account without manually re-writing them?

And the old-account outfit is planning to maintain their old servers indefinitely as they are moving to an entirely new IT setup, so I'm not going to lose that forwarding. Sorry about that "shutting down;" I meant shutting down their old setup and migrating to a new one, not going out of business entirely.

Also, it's not clear what "Manage Folder Subscriptions" means. Are those folders the ones at the icloud servers setup there? I am planning to have all mail come into the inbox there and do nothing with them there except be available for TB to download.

I try to stay un-enmeshed in e-mail (whence the desire for one account only) and to use only TB as a mail client. I suppose I could use a browser from other locations and other devices, but I'd rather not erode my otherwise healthy boundaries about the seductions of internet engagement.

Rather than use multiple accounts, I keep things sorted out into the Local Folders, which is why that matters to me...

Local Folders is a common resource and available for use with all or any of your email accounts. The confusion that arises is that all the folders under a POP account are also "local" by definition; these are stored on your computer.

In an account that uses IMAP, by default the messages are actually stored on the server and the mail client keeps a local cache copy for speed of searching and offline use. These caches are often mistaken for local folders too, but they are not intended to be persistent.

You can use filters to move messages from an IMAP account to Local Folders. POP accounts have a peculiarity in that in Thunderbird, they can be set to use a Global Inbox in Local Folders so all email arrives in the same one place. IMAP doesn't allow this, but you can instead move messages by filters, or you can use the Unified Folders view, which offers another way of amalgamating multiple accounts into one.

In IMAP, folders are set up on the server, and that structure is mirrored in the client. You can elect to view or not view ("subscribe to") certain folders. Of those you do choose to view, you can elect to have local copies or not ("synchronisation"). You're trading off convenience (messages always to hand) against time and disk space consumption (messages on the server don't use up your disk space, but take longer to access when you want to read them.)

You will almost certainly find that your provider offers POP, though he may not actively promote it. Setting up your new account to use POP will feel a whole lot more comfortable to you.

Thanks again, for your thoroughness. After wrestling with some demons, it's looking like I have newly-sent messages actually appearing on both platforms (TB and iCloud).

It turns out that icloud does not support POP at all, so after I leave my old account I'll have to move all my old nmci inbox messages into my new icloud inbox, which seems feasible, and generally sort things out. However, there's no TB-local "sent" box for the icloud IMAP account (at least that I can find). What am I missing?

Now how about those pesky filters? Can I copy them or do I have to re-create them manually?

As I age I am less and less facile with this technology that once I had mastered (when it seemed simpler, and probably was). I am so happy that others are maintaining mastery and so generously sharing it.