Mail, mail and blackmail. What does mail that your postmen brings have to do with chain mail that knights wore? What does either have to do with blackmail, where a villain extorts money to conceal your guilty secret? As it happens, none of these are related.
Mail handled by the post office (American and Scottish) is derived from an old english word for bag or pouch; it's really just the sack that the postman carries the letters around in, from old French for bag, wallet. That makes "mailbag" a redundant word, and "mailbox" a funny one. A knight's chain mail, by contrast, is from an unrelated old French and Latin word meaning mesh. So there was a time in middle English when you could have made a pun about the mail[bag] made of mail[=mesh].
Blackmail is a compound of black (which is almost always pejorative in compounds) plus an old Norse word meaning speech or agreement. Thus blackmail is an unfair, or forced agreement, or tribute; think "dirty deal." You can capture the essence of the blackmail "agreement" by picturing a heavily armed band of tough Norsemen showing up one morning at the gate of a defenseless english town and explaining that they're there to make an agreement for the town to pay them tribute--"protection" money, so to speak. The ultimate Indoeuropean root is *mod- which has the idea of to meet, assemble, and from this we also get "meet" and "moot." "Moot" is an interesting word for the twists and turns its meaning has taken. It started off meaning a meeting (such as a council), then came to mean debate or argument (such as "moot court") and now, as an adjective, has changed from meaning "debatable" to meaning "unimportant."