Mail on mail

A clever phrase!

"Mail on mail" could refer to a few different things, but here are a few possibilities:

1. Enclosed mail: In this context, "mail on mail" means that one piece of mail (e.g., an envelope) contains another piece of mail (e.g., a letter or package).
2. Layered packaging: This phrase could also describe a situation where multiple layers of packaging materials (e.g., boxes, bags, or envelopes) are used to protect and transport an item.
3. Meta-mail: In a more playful sense, "mail on mail" could be seen as a tongue-in-cheek reference to the concept of meta-mail, where the content of the mail is itself about mail or the act of sending mail.

If you have any more context or clarification about what you mean by "mail on mail," I'd be happy to try and help further!