You can select account settings and then select server settings and specify to leave messages on the server.

Thanks, but the problems with that is that the pop3 state is not saved on the server, but rather locally, so when I return from my trip and get "new" messages on the desktop after I use the other machine, I will get a profusion of messages that I have already processed and which got sorted into the folders on the machine that I read them on. Seems to me that leaving messages on the server only works if the server maintains state, and that would be IMAP, no?

Well, that wasn't part of your intended direction. Specifying to leave on server does achieve your goal, but accessing from different PCs does cause previously downloaded messages to download again. You're in a compromise situation. If you do not trust your ISP's server, your safest move (in my opinion) is to create a second account in IMAP and download everything (file>offline>download) and then copy all of it to local folders. That is, if you don't trust the ISP and if it goes bust, then you've lost everything. Then, you could continue to use the account from multiple PCs, knowing you had a full copy. And do backup if you take that route.

Thanks. I guess I wasn't detailed enough in the question.

I believe that the last time I tried to create an imap account on a server where I had the same credentials for a pop3 account, thunderbird either would not or could not do it, possibly because the server would not allow it. So then I run into problems converting from pop3 to imap, which I haven't investigated.

But the problem still remains that since I will always want to keep email locally (if not only locally), although I would not download the same email twice, it seems that I would still somehow have to sync my locally-stored email between the two machines. Or am I missing something?

My suggestion is to migrate to an ISP that you trust, as IMAP would automatically keep all devices in sync. In current situation, though, I suggest using POP and keeping all messages on server. That means you will download to each PC, but it does protect you. However, you also stated you wanted to be able to delete from one PC yet keep on another. That will always be problematic unless you download to both PCs first and then do selective deletes.

If emails are stored in server then you can use IMAP. Set up Imap accounts on each device and they display only what is on the server. So all imap accounts no matter what device - they all display the same data. But you cannot regard those accounts as an independent copy because they synchronise with server.

If you prefer to use POP : Pop accounts do not synchronise with anything. They download a copy of whatever is in Inbox that has not been previously download to that specific pop account. A 'popstate.dat' file located in the profile mail account folders keeps a record of what has been previously downloaded into that specific account on that computer.

If you need to download same emails into two completely separate computers that each have a pop account, then you need to have setting 'Leave message on server'. It does mean you can then download to two or more separate devices, but it would also mean organising emails twice. The other problem is there would be two completely independent 'Sent' folders

The alternative is to perform a copy of emails from one computer to another or just copy the entire profile, but , when you copy thunderbird from one computer to another it would overwrite anything on the second computer, thus losing any emails you had in Sent on second computer - if they were different. One compromise would be to only send from one specific computer, so the second computer only displays whatever the first sent.

• Exit Thunderbird
• Access Profile : C://User/User Account/ Appdata/Roaming/Thunderbird
• Quick way to access that location - see image below.
• Copy that Thunderbird folder (if you have several 'profile name' folders - it copies all)

Then on other computer:

• Exit Thunderbird
• Access Profile : C://User/User Account/ Appdata/Roaming/Thunderbird
• Delete that Thunderbird folder
• Paste the copied Thunderbird folder into the 'Roaming' Folder

This means you have identical copy when you start Thunderbird. However, it is important that you have exactly the same version of Thunderbird operating on both computers at the time of copying from one to the other.

As you are experiencing - although it works perfectly to have two separate pop downloads, it's not really designed for ease of use - that's why imap accounts were created to get around the problem.

As david points out - any important emails can always be 'copied' into 'Local Folders' then those are independent copies on your computer.

re :I believe that the last time I tried to create an imap account on a server where I had the same credentials for a pop3 account, thunderbird either would not or could not do it, possibly because the server would not allow it. So then I run into problems converting from pop3 to imap,

If you wanted to create an imap account then it would have to created as a new > existing mail account. You cannot simply modify a pop into an imap account. Some servers do require people to enabled Pop and/or Imap - this is done via the webmail account using a browser. Some servers require a slightly more complex set up if using Oauth as Authentication Method. But at this point, I have no idea what server you use for emails, so cannot really advise much more.

@david: ISP is Comcast, but I also use POP3 with a few other email providers. Not sure where I said I want to keep some emails on one machine but not another. I want both machines to be identical. The lack of trust extends to the possibility that they get hacked as well as their incompetence leading to lost email or misconfigured server or suddenly they don't accept my credentials.

@Toad-Hall Thanks for the extensive tutorial. Fortunately, I already knew most, but not all, of that.

In particular, I am wondering what specific incompatibiliities exist if using different versions of tbird, as long as they are not separated by the great divide, since I use only old versions since I absolutely hate the UI of newer versions and I don't need the new features. There is an issue, though, with the use of oauth required by some, such as Microsoft, where it seems to fail both with old and new versions of TB.

The way I work is that I have a large number of tb mail subfolders, and mail is automatically and manually sorted into the appropriate folder. Doing that a 2nd time for 3 weeks of email doesn't sound like big fun.

But, I think I have come up with a simple solution. Namely, keep all TB data on an external ntvme ssd drive plugged into the primary desktop, and when I want to travel, just move it from the one machine to the other (the laptop), as if it were a thumb drive. Seems like that would work fine as long as the versions of TB on the two machines are compatible. Am I wrong?

In the past, what I have done is copied the entire profile folders, but that takes time, and has to be done in advance. Then when I get back, copied the entire profile folders back. Again a bit tedious.

Thanks to all for all the suggestions and clarifying my understanding of details.

Is there a reasonable way to keep thunderbird data files in the cloud

No.

You might get some mileage from using Thunderbird portable from portable apps and you can carry it all on a portable disk for use at home or on the road. https://portableapps.com/apps/internet/thunderbird_portable