Unlike selecting "on" for "Auto mail checking" and clicking "OK", clicking "Check now", does not turn on automatic (check-after-interval) mail checking. It causes the Configuration and Email viewer to disappear (exit), launches a single check for mail, and causes the email viewer to reappear only if new mail is present.

Warning! If there is email in your mailbox when you click "Check now", but no new email is present, the email viewer will not reappear automatically.