Pull the message list downwards — swipe or drag from the top of the list and let go. This works on mobile and on desktop, and shows a spinner while BlueMail checks the server.
You usually do not need to. BlueMail fetches new mail on its own, according to each account's Push or Fetch setting — see notification methods. A manual pull is for when you want to check right now, or after being offline.
If pulling down brings nothing in and you expect mail, see emails are missing and not receiving push notifications.
