Messages app on macOS constantly downloading messages

Must say not experience this type of issue on 1 M4 Desktop, 1 M4 Laptop, 1 M2 Desktop not on aa iPhone 14 Pro

Each using the Latest version of macOS / iOS 26.1

This “Messages constantly downloading” loop on macOS could be an iCloud Messages sync corruption issue.

It most often means one of the following:

The iCloud Messages database on one device is corrupted.

A device is repeatedly re-uploading its entire message history.

There is an iMessage server mismatch caused by different OS versions.

A large/failed attachment is causing the sync engine to restart endlessly.

You said:

Your Mac mini (macOS Tahoe 26.1) had the issue first

Then your iMac (Ventura 13.7.8) started showing it

Then your MacBook on 26.1

This strongly suggests one machine is corrupting iCloud Messages and the others are syncing the bad state

Usually it’s the first machine that showed symptoms >> in your case, the Mac mini.

On the Mac mini (likely the culprit): Fully rebuild the Messages database

Turn off Messages in iCloud on ALL Macs

On each Mac:
Messages >>Settings >> iMessage >> Disable “Messages in iCloud”

Wait 5–10 minutes for the devices to disengage.

Before you attempt the below 

Always make a Time Machine Backup  before  proceeding 

Some suggestions appear here have been already mentioned earlier

Here are the most common causes and fixes for the "Mail constantly downloading messages" problem on macOS (Ventura through Sequoia/Tahoe 26.x). Since this suddenly started across multiple Macs, the issue is most likely server-side, mailbox index corruption, or an IMAP resync loop.

This problem is most common with:

iCloud Mail

Microsoft 365 / Outlook.com

If the issue started simultaneously on all Macs, your mail server may be constantly reporting changed message states, forcing IMAP to redownload.

Rebuild the Mailbox (not just “Reindex”)

Try this per mailbox:

Open Mail

Select the inbox for that email account

1. Go to Mailbox → Rebuild

On large mailboxes, this can take hours.

This forces Mail to recreate local caches without deleting your server mail.

Delete Mail’s Envelope Index (Full Mail Reset)

This one fixes 80% of endless-redownload problems — especially after OS upgrades.

Quit Mail

In finder >> Go to:

~/Library/Mail/V10

Delete these files:

Envelope Index

Envelope Index-shm

Envelope Index-wal

Restart Mail >> it will rebuild the index

This does not delete email, only the local cache.

Disable and Re-add the IMAP Account

Important: Do not delete the account from Internet Accounts. Just disable Mail access.

Open System Settings → Internet Accounts

Select your email account

Toggle Mail OFF, wait 30 seconds

Toggle Mail ON

This forces a clean IMAP resync without removing the login.

Check for a “Stuck” IMAP Folder

Mac Mail can get stuck when:

A folder has 10k+ messages

The server reports corrupted UID validity

There is a "phantom" folder on the server

Check using webmail for:

Very large "All Mail" (especially Gmail)

Old Archive folders with 50k+ messages

Duplicated folders (“Archive”, “Archive 1”, “Archive (2)”)

If you find any, cleaning them often stops the downloads.

iCloud-Specific Fix

If you use iCloud Mail:

Go to iCloud.com → Mail → Settings → Preferences → General

Click “Empty Trash” and “Compact Mailbox” (if available)

Then sign out of iCloud on one Mac and back in.

Third-Party Antivirus / VPN / Network Filters

macOS Mail is extremely sensitive to anything that intercepts IMAP.

Check if you have:

Avast / AVG

Norton 360

Bitdefender

CleanMyMac "Protection"

Cloudflare Warp

Little Snitch or LuLu filters

Temporarily disable → restart Mail → observe if the download stops.

No problems 👍

Let me get back to you

Though, do you still insist on " Don't I want iCloud ( Edited ) email messages turned on " ?

So who is / are your email account / accounts associated with < by name or names < ?

Do they all use the IMAP Protocol ?

OK, got that

Get back you in a moment or so

Just to get this out of the way before we dig deeper into this email messages issue

There are 4 Categories of Third Party Software / Services that are not needed 

Get rid of them via the Developers Instruction 

In no special order 

1 - Third Party 2 Way Firewalls 

like Lulu and Little Snitch 

2 - Commercial VPNs 

They may not be what one believes they are doing for the computer and what they actually are doing behind the scenes and unbeknownst to the user ( you ) 

They may also reduce your Internet Speeds by upwards of 30% 

https://gist.github.com/joepie91/5a9909939e6ce7d09e29

VPN: What you need to know - Apple Community

3 - Third Party Security Software 

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

The Operating System resides in a Sealed and Read Only Volume that cannot be opened by the User nor by Third Party Applications.

The Only thing this Antivirus software is protecting is the Bank Account of the Developers and for zero return to the User aside from the problems this software creates.

Security. Built right in

Mac app security enhancements

The Built in Security  is all that is required to protect the computer.

Protecting against malware in macOS

4 - Third Party Disk Cleaners / Optimizer 

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster. 

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a percentage on each sale. 

What the Application may do to the computer is up to the User to check this out before purchase

To put the Third Party Disk Cleaner / Optimizer in context and the damages it may have or has already done.

This type of applications can or will Muck Up your User Account ( Home Folder ) of this machine.

It does not touch the Operating System itself unless you consider your User Account ( Home Folder ) as part of the Operating System.

The Operating System may be hosed