Rum and champagne is an unusual combination but one which works well in this 1940s cocktail.
Add ice to a mixing jug.
Add rum, honey syrup and lime juice to the jug and stir thorouhgly.
Pour, unstrained, into a highball gass.
Top off with champagne.
Drinks historian David Wondrich has said this cocktail randomly popped up as if by "spontaneous generation". Its name though is easily explaned: "Way back when, airmail was the last word in getting something from point A to point B. And in fact, this one'll do just that to you, but quick. So quick that, if we were inclined to take such liberties, we'd rename it the 'Email.'"