Welcome to Whatbird @Margaret hubbardA snowy owl would be a tight fit in even the biggest of mailboxes. As @Kevin just said, if you could get a photo of the bird, that would really help in finding the right ID. If a photo isn't possible, more info will be needed. Size and shape of the bird AND the mailbox. What's the bird feeding on, seeds, fruits or berries, insects, rodents? Beak size and shape, leg and eye colour, they can all help narrow down your options but a photo just makes it so much easier, and usually more accurate.
EDIT- You'll get quicker responses if you post in the ID forum next time.