Pop does not sync. Only imap sync's.

Please explain. I thought emails were left on my server until I deleted them using whatever device I accessed them with. What in that regard does POP do? What do I do on my end, e.g., in Settings or Options, to get my desired result. Or is there another alternative?

I have never been easy with "Leave on server until I delete it" options in POP. What if you move a message to another folder? Or another account? At some point it must be possible to delete the copy on the server without also deleting the local copy. How is this distinction made? At what point does a message become detached from the server and assured of an independent existence?

If you want to work with your messages on multiple devices, then IMAP is much better suited.

Messages I want to keep I do move to a local folder on my DT. I have a long list of Keep-Some Name. If I move an email to a Keep file does it still remain on the server? Personally I prefer the idea of leaving messages I want to keep on my DT computer for long retention and on my NB computer only during a trip, rather than on the server. On the same topic how do I copy my Keep folder to my NB from my DT and vice versa. It should be a simple Copy/Paste operation, but I have not been able to make that work. Thanks in advance for your help.

Again, POP seems to me to be more complicated to use.

To make a permanent local copy I too would use Local Folders. But I think you'd have to copy the message to Local Folders then delete the original, at which point Thunderbird would send a delete request to the server. So that's two operations; Copy and Delete. I might be wrong. I don't know precisely which operation triggers the deletion at the server. It may be sufficient to move the message out of the Inbox.

With IMAP all you have to do is move the message to Local Folders; the fact that it has disappeared from the account folder would be sufficient to trigger removal at the server.

I actually store messages that I want to keep on the IMAP server; then they remain visible to whichever computer, phone or tablet I happen to be using. Older stuff that doesn't need to be so visible can then be moved out to Local Folders.

You can save individual messages to eml files on one machine and then import those files on the other machine, in both cases by drag and drop. For multiple messages/folders I'd use the ImportExportTools add-on. But there's no easy way to merge so you'd have to do tidying up and consolidation after moving. And you'll be fiddling about copying files to and from a memory stick, which all seems rather perverse when both machines are already potentially interconnected, with shared folders, through their connections to an IMAP server.

Thanks, but I need to think this over.

A halfway house; set up a new account using IMAP (try gmx) and add it to both NB and DT. Create folders "For DT" and "For NB". You can now copy messages for the other computer into the relevant folder in the new account, and on the other machine drag them to the appropriate local folder. You don't need to leave anything in the IMAP account for longer than it takes to get it moved. No sticks, no importing or exporting. Still two operations.

Use it for a while and you might start thinking "why bother to do the second operation and move them into the second computer?" It comes down to how cautious you are and how much you trust your data to an email provider. But since they hold your data for you between it arriving and you collecting it, usually for hours at a time, I don't see that leaving messages on a server for extended periods introduces any greater risk.

Other alternatives; use VNC or TeamViewer, or Microsoft's own Remote Desktop tools to control the NB from the DT. You should at the very least be able to make eml files from messages on the NB, use the built in-file transfer mechanism to teleport them to the DT and then drag them back into the DT's email client, and vice versa for the opposite direction. But still a whole lot more work than just using IMAP. ;-)

In fact, you probably could work just with the built in "homegroup" networking facility; share a folder on each machine to the other. Drag and drop email messages into the appropriate shared folder. Then at the other computer, drag them into the email client. Still more work than IMAP, but no new software, just using already available tools.

This is all just reinventing a wheel. We're contriving methods to do just what IMAP already does.