Here's why:
* The "ai" in "mail" makes a short "a" sound, like the "a" in "cat". It's not a long "a" sound like the "a" in "cake".
* Long vowel sounds are typically associated with a single vowel followed by a silent "e" at the end of the word. This is not the case with "mail".
So, "mail" is a short vowel word.