Mail.app constantly re-downloading hundreds of IMAP messages

When you "Rebuild" a Mailbox, it will say it is downloading messages. It is actually just scanning every email on disk and writing the information into its database. That may be what you are seeing.

It obviously shouldn't need to do that, either, but it may be finding the database corrupt and is rebuilding it.

If it is having trouble writing to the database, deleting it and letting it recreate the database may solve the problem.

Its name is "Envelope Index" and there are two other database log files, -shm, -wal.

They are located in ~/Library/Mail/V8/MailData/

With Mail closed, delete all three, then open Mail. You should see the Downloading messages status in the Activity window.

If your user Library is hidden (default), you can open it by holding down the Option key and selecting Library from the Go menu in Finder.

I have noticed this with MS-Exchange accounts. I don't see it with GMAIL or POP accounts. With the Exchange accounts, I see this behavior on multiple Macs running everything from El Capitan (10.11.6) through Catalina (10.15.7).

I believe it may have something to do with the way the Exchange server tracks changes or messages, and the Apple Mail interface.

However, one of the Macs I use on Catalina uses MS-Outlook, not Apple Mail, for the Exchange emails. The Apple Mail on that Mac is still used for GMAIL and the POP account, but it comes up and updates instantly without this undesired behavior. I see it only with Exchange and on the other Macs that aren't using Outlook but are using Apple Mail.

You might consider switching to Outlook if you are using Exchange emails. Outlook does not exhibit this behavior at all. At one point I even had both Outlook and Apple Mail accessing the Exchange IMAP emails on the same computer. Apple Mail behaved in the way you describe, but Outlook did not do all that downloading.

Given your situation with the remote location and bandwidth limitations, I think this is something of a showstopper for you and it might be worth trying Outlook. But if your IMAP account is not an Exchange one, I'm not sure if what I am suggesting is the best approach.

Another possible thing to investigate -- do you keep very large Sent or Deleted (Trash) mail folders? Are there a large number of message in those folders, or is your In Box extremely large? You may want to experiment with moving and storing messages to other folders from the In Box to keep it very small, and also to empty the Trash and shrink down the Sent folder. I have a theory which I have not proven yet that very large folders may get resorted or reshuffled when additional messages are added and this might make some email programs (like Apple Mail) do a lot of "work" with them, just because a few new messages have been added. So if you had 10,000 messages in your In Box, if you were file them into 20 other folders that you create (these folders can be set up by date span, or by topic, or by sender ...), maybe 50 messages per folder, you can still access them easily but your In Box will be kept very small and when something changes in it, Apple Mail will handle that quite quickly without downloading a lot of stuff.

If Apple is unable to help you find what is causing this on your computer, I would suggest switching to the webmail interface which will not involve any bulk downloading.

One thing I'm not sure about. You said the account was once POP. My understanding is you can't convert an account from one type to another, and you would need to create a new account. Does the POP and IMAP account information reside separately? If they are somehow still connected, I'm not sure what would happen when the POP messages do not exist on the server (if it even checks). I have never tested with that situation.

It builds from what is stored on disk, which is what I was trying to describe in the first part of my post. I have no idea why it says "downloading" when it does the rebuild.

Sorry, I'm not sure what else to do. Have you removed the account and added it back?

If it is IMAP, all of the messages should be on the server.

P.S. One more thing -- if you were to decide to switch to Outlook for Exchange emails, you can use Outlook for all new Exchange emails and keep Mail as an simply a static storage for everything earlier. You would disable checking and downloading from that account for Apple Mail, since Outlook would be used for all new messages. All your existing emails would still be accessible in your Apple Mail, however. This would avoid the costly (bandwidth-wise) step of trying to "convert" from Apple Mail to Outlook mail (I'm not even sure that is even feasible).

By the way, Outlook offers an option to only download attachments if they are below a certain size. Larger attachments are kept on the server and are only downloaded when you tell it to. That also saves on bandwidth if you have tight constraints.