Networking :: Connections In Virtual Machine Getting Dropped?

I've forwarded incoming connections on port 25 to my virtual machine with the following commands:

sudo iptables -t nat -A PREROUTING -p tcp -i eth0 -d 10.1.1.3 --dport 25 -j DNAT --to 192.168.56.101:25

sudo iptables -A FORWARD -p tcp -i eth0 -d 192.168.56.101 --dport 25 -j ACCEPT

The strange thing is the connections coming through in the virtual machine seem to be getting dropped. A port scan from the internet says the port is closed, but it is not the case as I can see the connection coming through. As you've probably worked out, I am running a mail server. When I send myself test mail, the connections from the senders mail server also gets dropped.