What does mail skin mean in English?

“Mail skin” isn’t a standard English phrase, so it most likely comes from a typo or a mistranslation. If you saw mail and skin separately, “skin” usually means the outer covering of a person or thing, or it can be used as a verb meaning to scrape the skin off something.

Most likely meanings

• Skin : the outer layer of a person, animal, fruit, or object.

• Mail : can mean postal letters, or in older usage, a type of armor made of linked metal rings.
• Mail skin together does not form a normal English expression, so the intended meaning depends on the original sentence.

If you meant a phrase

• “by the skin of one’s teeth” means barely managing something.

• “skin mail” or “mail skin” could also be a mistaken rendering of a slang term, a product name, or a phrase from another language.

The safest translation is: “mail skin” has no common meaning in English on its own. If you share the full sentence, I can translate it more accurately.