For security reasons and power reasons a mail server do not have to run on a router !!!!!
(or a virus scanning engine!!!!!!)
Please google for some explanation ......
These ASUS devices are designed to run as a router !!!!
(with some little extensions.....)
If you really want a mailserver put it behind the ASUS device and configure properly a port forwarding.
I strongly advice you, if you are not sure what you are doing, and it seems so, because you are asking for these features on the small ASUS device,
please install a seperate server behind the ASUS router.
I know that the ipkg system is very easy to use, but you have to understand a little bit more of cpu and memory consumption;
and in addition LINUX organisation of swap and TCP connection rules and so on....
Just my to cents....