My parents have a mail slot in the front door of their apartment and wanted to know how to keep the cold air from coming through. Unfortunately, short of taking it out and boarding it up , it appears there is not much you can do to make these energy efficient. There are newer mail slots that can be used but they cost anywhere from $50 to $100. In an apartment that just isn't an option. I did find one thing that may be an option. This is a magnetic seal that covers the inside of the slot and only costs around $20. This is the company's website, but other it may be sold other places as well.