How to expose ports with iptables?

I have two containers both are postfix container,Container A has been create without exposing port 25 and container B with expose port 25. So i tried to mimic same iptables like the one generated for container B on my host machine but no luck. Is there anything else i should configure before i could get access to port 25 on container A. Also any easy way to expose ports on already created container ?

here are my iptables rules:

iptables -t nat -A DOCKER -p tcp  --dport 25 -j DNAT  --to-destination
iptables -t nat -A POSTROUTING -p tcp -s -d  --dport 25 -j MASQUERADE
iptables -A DOCKER -p tcp -d --dport 25 -j ACCEPT