My Macbook constantly shows downloading messages, currently 11,069.

The following might help:

Quit Mail. Restart the computer holding down the shift key, waiting some ~ minutes until the login screen appears. You should see "Safe Boot" on the login screen and/or at the top of the screen after you log in. Let the computer complete the initialization of the finder window, which may take longer because this is Safe Boot, which makes things slower. Safe Boot removes some caches, the Mac uses caches to "remember things" and operate more quickly, but in this case it is remembering something (a message count for download) that you want it to forget.

Now restart normally. Is the downloading many messages indicator still there?

I think it sometimes get stuck with an incorrect value and at that point is not informing you what Mail is actually doing. For instance, when that happens, you can look in Activity Monitor and verify that there is not much Network activity -- downloading 11,069 messages would show a lot of Network Activity normally. It's probably a bug but largely a cosmetic one, I think.

Sometimes just rebooting normally ends that incorrect downloading display, but the Safe Boot forcefully has the Mac clear out lots of caches where sometimes the stuck value is stored.

Hello Cnick100,

Thank you for using Apple Support Communities. Based on your post, the Mail app is constantly downloading messages and never completes. We'll do our best to assist with this.

First, we'd try out steps in: If you can’t send or receive email on your Mac

Next we'd confirm you're connected to a known good working Wi-Fi network. That you aren't filtering the Mail app content.

Are you able to send emails successfully?

If you go up to the Mail app in the menu bar, tap on the Window menu > Connection Doctor. Are all of your email accounts online? Are any offline?

Hoping to hear back, take care.