A letter by mail or in mail

A clever play on words!

In this case, I'm assuming you're asking about the phrase "a letter by mail" versus "in mail".

A letter by mail typically refers to a letter that is sent through the postal service, also known as snail mail. This means that the letter is physically carried by mail carriers from the sender to the recipient.

In mail, on the other hand, is a phrase that is often used to refer to a letter or package that is contained within a larger package or envelope, often referred to as a "mail packet" or "mail bundle". This could include a collection of letters, packages, or other items that are being sent together through the postal service.

So, to summarize: