View Mail activity
Monitor Mail’s activities, such as the receipt of large messages.
See status and activities: Mail shows some status information at the end of the Favorites bar—for example, if an account is offline.
Mail shows some activities at the bottom of the Mail sidebar. For example, it shows messages being downloaded or when it’s paused a large download to save energy while your Mac is on battery power (you can click the button to resume the download).
Monitor activities: Choose Window > Activity to open the Activity window.