"Email ping-pong" is the act of email correspondence between two people getting out of hand or control.
It usually starts of with a comment, opinion or observation that becomes misunderstood or misinterpreted by the recipient. The recipient then responds with comment, opinion or observation...and so on. It is not uncommon for participants to ping-pong 10, 20 times or more before they realise they haven't got a clue what started it all in the first place!
The act of email ping-pong might start quite innocently!
Ping - Dear ^+*%$#,I noticed blah, blah, blah...
Pong - Dear *£*'@}$, As you should be aware... (how dare you tell me what I should do)
Ping - Dear ^+*%$#, I am perfectly aware... (Get your head out of your arse)