said by Donuts :There is no way to change the default port on the WNDR4300 either. All it asks for is the server address. I was rather frustrated after I learned that knowing full well that port 25 was not an option. I tried the : specification on the server name - and it worked.
Might be worth a shot to try it.
Doesn't work with the R6300, tried that first, confirmed by Netgear "Support".