If it's a POP account then, yes, it is local and doesn't care what happens to a copy, if any, on the server. (But I would ask how you will manage what is on the server if you are downloading via POP but leaving a copy on the server?)

I frequently advise users to move messages to Local Folders for safe keeping. Doing this detaches such messages from an IMAP server, so the Local Folders copy doesn't go if the server copy copy goes.

I often advise doing the same for a POP account, particularly if the account is to be closed. Whilst the messages would be safe in the folders of an account configured to use POP, the user would get irritating messages if the account tried to connect to its server. So if you move the messages to somewhere safe, you can delete the original POP-connected account to avoid the errors due to connection failures.

Since your POP-connected accounts are active and in use, you don't need to be moving your messages to Local Folders. Indeed , it does get messy,

One of the main motivations for using Local Folders is a preference to have everything in one place. Some users will use the Global Inbox to aggregate messages for all their POP-based accounts, and in doing so, they would usually set all accounts to also use common Sent, Trash etc folders in Local Folders.

Thanks for such a cogent and swift reply, Zenos.

I understand the use of custom Local Folders to aggregate messages like social media notifications (things you might want to hang on to for a while but are rarely going to reply to directly). But doing the same for messages that are part of a conversation gets very messy.

You say: "I would ask how you will manage what is on the server if you are downloading via POP but leaving a copy on the server?"

In my case, two users need to be able to collect the same messages from the same server accounts, but to different Macs at different ('staggered') times. My understanding is that if either of us used IMAP then the first to download any mesage would always delete it from the server as a result of the sync 'talkback' from their client app. (Am I right?)

My solution is to set a 'leave on server for 'x' days' period under every relevant TB Account that allows both users a known and reasonable period to pick up all messages (14 - 21 days is usually ok).

I've never found a better way than this to manage multiple downloads of the same messages from the same server folders/accounts. If you know of one, please let me know!

Thanks again for the reassuring info on TB Account Inboxes under POP.

There are some misconceptions in you last post, and I will try and address them.

First, despite names and labels ALL folders involved in a pop mail account are Local folders. As you are no doubt aware POP downloads mail from the inbox and nothing else. This causes problems for some people where the providers server undertakes spam processing and moves new mail to a spam folder. It never gets downloaded because it is in the incorrect folder. It is something everyone using pop needs to be cognizant of.

Once you accept that POP is truly a download of discrete email with no link to the server you come to understand why all POP accounts are "local" so regardless of the location your store them, under the hood Thunderbird treats then exactly the same.

In my personal use of Thunderbird my primary accounts are in fact POP and I use the global inbox feature to aggregate incoming mail from all of them to the inbox on local folders. I is how I prefer to do most of my email as the incoming account is not important to how I process my mail.

IMAP however is something you need to explore further.

IMAP is a fully synchronized mail protocol with the server store as the canonical store. So if 2 of 20 clients synchronize with that account on the server each client will show the same emails, in the same folders with the same tags and read/unread status. This is ideal for small sales teams etc as a number of people get the email, as soon as someone reads it everyone get notified the mail is read. A reply is also synchronized so everyone get a copy. This can be particularly powerful when Thunderbird is set to place copies of replied in the same folder as the mail being replies and threaded mail views are used. Suddenly everyone not only has a copy of the reply, but it is essentially "pinned" to the original.

IMAP also allows for folders to be subscribed and unsubscribes, so not all server side folders need to be seen by all of the group in their email.

Now I have talked it up. The reality check. If someone deleted a mail, everyone get it deleted for them. So one idiot can do a lot of damage. It is also true that the local copies of email in IMAP stores in Thunderbird are best considered a cache to speed display and liable to deletion as soon as the computer can not synchronize the account with the server. This can be an issue where connectivity is spotty or you have security software that is prone to blocking or crashing connectivity.

POP on the other hand is all local, so you get none of the benefits of the synchronization, but you also get the stability of having permanent copies in multiple locations. Which is better for a given task or group is difficult to say and personal preference really does play an important part.

My IMAP account for instance are also the account I synchronize with my phone. So I do a little email on the phone and have that synchronized back to Thunderbird. Personally I feel I have the best of both worlds, but I do have the responsibility of making my own backups as there is nothing n the "cloud" to fall back on.

I do suggest you look at the information here https://support.mozilla.org/en-US/kb/glossary-terms-including-types-accounts#w_mail-accounts

It is a short summary of IMAP and POP with links to further information should you want to go further.

Thanks for the extra detail, Matt.

You confirm the main points that Zeno had helped me with: 1) POP is "all local". Under POP the client app does not sync with the server except in the very basic sense of only downloading messages not previously downloaded; 2) under POP, what are called 'Local Folders' in TB are no different from Account 'Inboxes' in their relation to the server.

I may indeed still have some misconceptions concerning the details of IMAP synchronisation, but I think my understanding of what I need and why is now pretty sound!

Matt said

Now I have talked it up. The reality check. If someone deleted a mail, everyone get it deleted for them. So one idiot can do a lot of damage.

My use case is different from Matt's. I wish to be able to see and work with my email at work, at home and when travelling. I do this on a variety of computers, a phone and a tablet. POP is of no use to me whatsoever. If I do any filing or re-organisation of my email and its folders, those changes ripple through to all devices. I don't need to repeat that work on each device. If I do delete a message I want to have to delete it only once.

My main use for Local Folders is for Saved Searches which give me aggregations, such as unread messages, recent messages, project-specific messages and so on. Because I have a mix here at work of personal and private messages, unlike Matt I don't want them blended together. Even so, I use tools that warn me if I might be using an inappropriate identity to send a message.

You are like many small businesses, trying to manage your costs and making do with what is to hand, when really for the situation you describe, you could benefit from running your own mail server and using its facilities to use communal mailboxes, delegated accounts, shared folders and so on.

In my case, two users need to be able to collect the same messages from the same server accounts, but to different Macs at different ('staggered') times. My understanding is that if either of us used IMAP then the first to download any mesage would always delete it from the server as a result of the sync 'talkback' from their client app. (Am I right?)