I can't help with the email function, but I've managed to get it working with schemingMind (a site which is exclusively correspondence chess with very different time controls from chess.com) which supports xfcc. There's no xfcc support in chess.com - see http://www.chess.com/forum/view/community/xfcc-support. If you want a bit of help with xfcc let me know. And sorry, I'm not too interested in engine v engine.
Not much help i suppose, but at least you've got a reply 
Can somebody please explain aboud the correspondence function?
As far as i understand, there are 2 options, through a xfcc server, or through email.
If this is correct, does anyone know how to set up the email function, and would someone be interested in playing a engine vs engine game with me using this function?