zen mail 

1) An e-mail with no message text or subject line.

2) An e-mail response with no message text.

"I clicked 'SEND' without thinking and only sent some zen mail."

zen mail by Ken Bergenham January 27, 2004

zenmail 

an e-mail message in which the sender refers to an attachment that has not, in fact, been attached. (Thus subscribing to the zen doctrine of non-attachment).

"Oops -- here's the PDF I promised, with apologies for my previous zenmail."

zenmail by Jehosophat February 25, 2009