We can Modify Connectors for Receiving Internet Mail on different port apart from port 25 through your spam filtering/ISP provider.
This step applies to Exchange 2007/2010/2013. It is always a best practice to have this kind of setup so the spammers will not be able to intrude in our network and perform a directory harvest attack,reverse NDR attack etc.., and we can prevent spam emails circulating in our environment.
Perform the following thing to achieve this task.
3) Change the port to your desired number on which you need to receive emails from them.
4) Disable the default receive connector since it’s not required anymore.
So the mail-flow for inbound will be in the following type
From Internet – Mails comes to your ISP/smart host – ISP delivers emails to your firewall on different port – then it comes to exchange server
For sending emails to the internet it would be very easy
Just create a send connector and smart host it to your (ISP/spam-filtering provider) IP address so that all the internet emails would be delivered to desired port to your (ISP/spam-filtering provider).
Outbound From Exchange – Email goes to your (ISP/Spam filtering provider) on a different port – Mail gets delivered to the internet user on standard port 25
Make sure that all the port numbers that you have configured to send/receive emails through your Spam filtering provider have been opened both inbound and outbound on your corporate and perimeter firewall.
Also refer – http://social.technet.microsoft.com/wiki/contents/articles/29577.modifying-connectors-for-sendingreceiving-internet-mails-on-different-port-apart-from-port-25-through-your-spam-filteringisp-provider.aspx