In mail or on mail

A subtle distinction!

"In mail" typically refers to something that is inside or contained within an envelope or package, such as a letter or package contents.

"On mail" typically refers to something that is written or printed on the outside of an envelope or package, such as an address label or a stamp.

For example:

• "I received a package in the mail yesterday." (The package was inside the mail.)
• "I need to write the address on the mail before I send it." (The address will be written on the outside of the mail.)

I hope that helps clarify the difference!