I had an issue where I would receive an new email notifications when there was no mail so I made envelope tray icon hidden but I didn’t know how to make reappear. So I am posting this because I want to share the knowledge.
Go to Tools – Options in Outlook. Choose “Email Options” on the tab labeled “Preferences.” Then choose “Advanced Email Options.” In this box, go to the
section labeled “When new items arrive in my Inbox” and check the box labeled “Show an envelope icon in the notification area.” When you choose Hide
Envelope in the system tray it essentially un-checks this choice in the email options.
