Have all mail get sent from one ipaddress on your server:
iptables -t nat -A POSTROUTING -p tcp --dport 25 -j SNAT --to-source IP_YOU_WANT_TO_USE
Have all mail get sent from one ipaddress on your server:
iptables -t nat -A POSTROUTING -p tcp --dport 25 -j SNAT --to-source IP_YOU_WANT_TO_USE