Gabriel

I have never tried that myself, but my suspicion is that the firewall rules will not allow it to work.  However, its very easy to test it yourself! 

I can't see a reason why you would want to do this?  Why not simply use secondary MX pointers in your DNS zone?  That way emails go direct to the secondary server when they need to...and you don't have any dependence on the endian's internet connection being available.