I think you may consider where the outbound trafic are going to, it go from 1.1.1.1 port 25 to ? you have to mach this trafic -and only this- and send it to 6.6.6.6
May be --sport 25 are enought.
Since you want to change the destination of the traffic, you need DNAT, not SNAT. You also need to match the specific packets you want to DNAT (i.e. destined for port 25).
iptables --table nat --append POSTROUTING --out-interface eth0 --protocol tcp --dport 25 --jump DNAT --to-destination 6.6.6.6:2500