Fortigate with Email Server behind

Hi All.,

for example, these are my network details (dummy) :

Wan IP  : 1.2.3.4

Lan IP : 202.12.22.12 - 202.12.22.28 (these are my public) one of whichis our email server.

My email server is connected to one of the Lan port (202.12.22.15). How can i make the default outgoing ip to be the same as the mail server ip address? 

Because right now, every time the user sends an email message, the ip address that is being captured at the email header is my Wan IP (1.2.3.4). The problem here is that, whenever our wan ip gets blacklisted, in will blacklist all domains connected to it.