Well, an "empty" email could mean that there were no words in the body of the email. I think "meaningless" might be better, or "pointless", which implies that the communication had no point, no matter how long it was.
I'm with JamesM -- "void" or "empty" would be an email with no text in the body, as people occaisionally send when their mail program is playing up. Plus, I like James' suggestions, particularly "book-length".
I'd add "over-long" to his list; and also, in the actual case I'd try for a longer and funnier (I'd fail, of course) way of putting it.