This is a just adding a point of extreme and uncontrolled air leakage to the house. Also consider the fun little critters that come in through these mail slots. As a child growing up, I lived in a house with a mail slot on the door and there was always an unwelcome bug coming through. Heaven forbid the mailman leave the mail into slot and just leave the opening open.

Have you consider a [gorgeous mailbox[(https://www.houzz.com/products/contemporary-mailboxes-prbr1-br~t_685~s_2103) instead? There are also built in mailboxes. I am not sure what your style preference is but there are so many choices out there.

Good luck!

My father installed a mail slot in a wall. He made a ramp from plexiglas, from the slot to a wood box. The wood mail box was in their garage, the slot had a flap.

My folks traveled and having this mail set up they did not have to ask neighbors to pickup their mail, and they did not have to worry about mail theft. Mail and package theft is a big problem where my parents lived and my location too.

Living in the city, I've got the luxury of to-the-door mail delivery.
All the tract homes in my 1950's subdivision have a mail slot in the
exterior wall. The inside is covered with a little wooden door that
snaps shut. Depending on the floor plan, the door may be next to the
front door - visible in the living room, or, in my case, it's hidden in
the front coat closet. The outside slot is vertical, not horizontal. The
homes are 8" masonry construction with plaster interior walls. The
interior of the slot is smooth since it is lined with thin metal
sheeting of some sort. Perhaps an experienced carpenter or cabinet maker could make a nice door to cover the opening for you.

As 'wishiwereintheup' stated, we also had a wall slot with a door in our St Louis Mo home. The season's are frigid to scorching hot. The county required block and brick built houses at the time- thick exterior walls. No problems or bugs. House was built in 1961.

Also, the slot on my house is not straight through. It's offset - something like the following as a side view. Has more capacity that way.

Houses in many, many cities have had these for decades. Some are in the front door, some are too the side and the mail drops into a hall closet. I had one in St Louis and also in LA - loved them both and no problems with either. Wish I had one now when the weather is too bad for an old lady to venture out to the mail box.

I had one of those, too. Just be sure you have some kind of weatherstrip around the interior door. Ours didn't and you could feel the draft!

A different problem is one my relative has: not all the mail can go through the slot, especially when it isn't straight. Magazines and large or stiff envelopes can't go through the turn.

Here is oldie but a goodie..

The back side..

Google "through wall mail box". UPS and FedEx still leave packages outside the front door...

Also, check with the postmaster. Unless you already have a mail slot in an existing structure, you may be out of luck getting them to deliver mail there. My parent's house was built about a year too late on the street. While their three next door neighbors have mail slots, they have a street box because the PO doesn't approve the slots anymore.