assuming you have a static ip address, i think all you have to do is create an A record and point your mail (MX record) to that address.
ex.(you want to have user@xxxxx.com recived by your es mail server)
mail.xxxxx.com IN A 123.456.789.123 (this is your es box ip address)
xxxxx.com IN MX 10 mail.xxxxx.com
hth,
Iwan