Drafts can be a nuisance in any environment, but they can be particularly troublesome in enclosed spaces such as mail slots. Cold air drafts can cause insulation to become loose, providing an opening for cold air to enter the slot and chill the mail.
In extreme cases, drafts can cause the closure of the slot entirely, preventing letters from being delivered. .
The best way to avoid drafts in mail slots is to keep the space well-insulated. This can be done by installing a well-sealed ceiling, using weatherproofing materials on the interior walls, or by installing a properly sized window.
If drafts are still an issue, consider installing a door that opens into the outside world.