Looking on the internet deeply has found these results:

in your mailbox is the most popular phrase on the web.

on your mailbox 16,980,000 results on the web	in your mailbox 17,030,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
This thing could be activated by your mailbox key. You know she's the one who wrote that on your mailbox, right? I left a flyer in your mailbox. I mean, what does it say on your mailbox? On motorcycle decapitations in your mailbox. Excuse me, your mailbox is... Drop this in the mailbox on your way out. I saw your name on the mailbox.	I'm so sorry. I forgot to get a release form from you, so I ran it by and put it in your mailbox. Tell me, how was it special with the girl who puked in your mailbox or the girl who says she doesn't wear shoes Shouldn't keep your key in your mailbox. I always turn my phone off before I put it in your mailbox. On motorcycle decapitations in your mailbox. Yes, they were in your mailbox. I left a flyer in your mailbox. I could also leave it in your mailbox.

on your mailbox

16,980,000 results on the web

in your mailbox

17,030,000 results on the web

More popular!

I'm so sorry. I forgot to get a release form from you, so I ran it by and put it in your mailbox.
Tell me, how was it special with the girl who puked in your mailbox or the girl who says she doesn't wear shoes
Shouldn't keep your key in your mailbox.
I always turn my phone off before I put it in your mailbox.
On motorcycle decapitations in your mailbox.
Yes, they were in your mailbox.
I left a flyer in your mailbox.
I could also leave it in your mailbox.