Cisco ASA routing

I have a cisco ASA 5510 firewall.
have 3 physical interfaces on it right now

corp-192.168.15.1/24   (192.168.15.2 is corp router)
inside-192.168.5.1/24
dmz-192.168.102.1/24

we have another network between a router on the corporate network that has a network on the same address as one of our physical networks (192.168.5.x/24).  It has an SMTP server that we want a server on our DMZ network to be able to use.

when we send a request from 192.168.102.99 (DMZ network) to 192.168.5.6 (SMTP) server, it is routed to the physical interface on inside network.  

How can we override this and any requests made from 192.168.102.99 to 192.168.5.6 using port 25, get routed out the corp network instead of trying to go to the inside network by default.

was thinking a route statement and/or a static statement is what i need

route outside 192.168.5.6 255.255.255.255 192.168.15.2 1

or do need static like below

static (DMZ,outside) 192.168.5.6 192.168.5.6 netmask 255.255.255.255

or need both?

thanks