View Mail activity on Mac
You can check the status of some Mail activities, such as sending or receiving large emails.
In the Mail app on your Mac, choose Window > Activity.
In the Activity window that appears, monitor the progress of Mail activities.
Mail also shows some activities at the bottom of the Mail sidebar. For example, it shows messages being downloaded or when a large download has been paused to save energy while your Mac is on battery power (you can click the button to resume the download).
You can see some status information at the end of the Favorites bar—for example, if an account is offline.