“Chain mail” (amour) shouldn’t be confused with postal service mail or email, but there is a riddle:
 
Q: How do knights communicate?
A: Chain mail.
 
The joke was posted on Twitter by “bowl in a china shop” on July 23, 2011.
   
     
Wikipedia: Chain mail
Chain mail (often just mail or sometimes chainmail) is a type of armour consisting of small metal rings linked together in a pattern to form a mesh. It was generally A common military use between the 3rd century BC and the 14th century AD. A coat of this armour is often referred to as a hauberk, and sometimes a byrnie.
 
Twitter
bowl in a china shop
@humourheadshark
@tylerjackbutler
Q: how do knights communicate? A: chain mail #raginggroaners #beerswesnesday?
5:55 PM · Jul 23, 2011·Twitter for Android
   
Twitter 
Russ Hoe
@Russlemania
Why do knights send spam to each other?
Because each of them has their own chainmail.
7:49 AM · May 29, 2012·TweetDeck
 
Twitter
Nerd
@nerdydadjokes
How do knights communicate with each other? They send a chain mail!
12:35 PM · Dec 28, 2015·Twitter for iPhone
     
Twitter
Victoria
@victooriaaaah
Do knights communicate via chainmail?
5:26 PM · Jan 15, 2016·Twitter for iPhone
 
Twitter
Joe
@josephoooo
How do Knights communicate?
They use chain mail
#KingOfDadJokes #BudweiserSweepstakes
@budweiserusa
11:44 PM · Jun 21, 2020·Twitter for Android
 
Reddit—Dadjokes
Posted by u/HellsJuggernaut July 5, 2020
How do Knights communicate ?
Chain mail