Add the clean IP to the server.
Edit the firewall to NAT all connections FROM port 25 to use the new IP.
# iptables -t nat -A POSTROUTING -p tcp --dport 25 -j SNAT --to-source NEW.IPA.DDR.ESS # service iptables save
Ensure that the IP is on eth1
or…
# iptables -t nat -A POSTROUTING -o eth1 -p tcp -j SNAP --dport 25 --to-source 123.45.6.7