if i am switching exim default ip through /etc/mailhelo,
should i add
Just to make sure, /etc/mailhelo is not the exact place where you can switch the default IP address of Exim. It is /etc/mailips where we change the mail interface IP address; /etc/mailhelo is used to adjust Exim's HELO message. And yes, using "hostname" is good for server wide domains ( that is for * ).