Allow IP to relay 3rd party emails

I am attempting to allow our exchange server to relay messages from account X and send it from our server. The reason is because the email being sent to a client is demo@quosal.com and we would like our internal email to be used. How would i allow certains IP's to be relayed and what port would be used to reach the exchange server?

The IP addresses for the order porter server are:

(These are the ip addresses that you need to allow your SMTP server to relay Order Porter emails. If this is not set, then the order porter emails will not be sent out.)

example : a public email address

In addition, the main email server for fall back emails comes from quosal.com directly. That IP address range is (public ip)