Mac Mail Not Downloading New Emails

This must be frustrating but there are a few things that you can do to at least try to fix this is. First make sure that you haven’t exceeded your storage limit in iCloud, this won’t back up your emails and therefor you won’t see any new ones, Check your iCloud storage on any device - Apple Support. Also take a look at which filters you are using , this can sometimes restrict what you are able to see based on the category each specific email is in. Filter emails in Mail on Mac - Apple Support. To see each one make sure you are in/have the “primary” filter turned on.

If the issue persists don’t hesitate to ask, good luck!

Have you tried removing the account from Mail and re-adding it?

Actually you can, but it's a bit drastic since you basically have to remove your Apple Account from your Mac. Perhaps this may help:

If you can’t send or receive email on Mac - Apple Support

Regards.

FWIW, I also have an M4 Mac mini running macOS 15.2 Sequoia and Mail has no problems with my iCloud account nor with my non-Apple mail accounts. So this is not a universal problem, at any rate.

Regards.