There are a lot of letterbox clues that have cryptograms you need to solve.

A cryptogram is a substitution cipher, which means that a different letter or symbol is substituted for the regular alphabet letter.

Like this:
3ls83# dl1i th3s

(Don’t try to solve that, I just made it up).

I don’t like to cheat in solving clues, but I have no problem using tools that take the busy work out of it.

I found a neat online cryptogram helper. It automatically substitutes letters for you without solving anything for you.

Here are some examples of letterboxes that use cryptograms:
A Midsummer Night’s Dream Letterbox
Dancing Men.