Mail in System Prefrences > iCloud only refers to your iCloud email accout.
This is your email ending in one (/some/all) of the following:
@iclould.com, @me.com, @mac.com
So if you use your email account provided with your iCloud account enabling it in iClould allows you to access it within the Mail app and it will keep in sync.
However this is only your iCloud account. You may have other email account that you find are not syncing. To make sure other email account sync across devices you want to be using IMAP or in some cases Exchange.
If you go to prefrences in Mail and then accounts you’ll be able to see your email accounts and the account type below. You’ll want it to say IMAP. But if you see POP you won’t have the email syncing across your devices. In order to use IMAP you’ll need to see if your email provider supports it (most do). If it does it would be a case of updating the mail settings. For this you may need to re-add the account in system preferences > internet accounts.
If you have been using IMAP but it is now not syncing you may have an issue with your email account on your machines reaching the email server in which case check your mail settings and try and update them.
This post was modified 7 months ago by elliotjpb