Post that once provided personal protection (4)
This might be a double definition.
'post that once provided personal' is the first definition.
'mail' can be an answer for 'post' (synonyms). I'm not certain of the remainder of the definition.
'protection' is the second definition.
(mail is a kind of protection)
Can you help me to learn more?
(Other definitions for mail that I've seen before include "American post from Lima", "Flexible armour", "Form of armour; post", "Post in America", "Eg, letters".)