Question : Email security between my Exchange Server and a third party Web mail domain

Hi, we have a working Windows 2007 Exchange server for our main ABCD.com business email name. Everything works great.
 Our CFO purchased a new domain 1234.com with a provider for a new Web site and created 1234.com email accounts for certain users ( 90 users) .  QUESTION:  the CFO asked me to see if I can configure our Exchange Server to send/receive email between the two domains "Securely"  (and is not too clear on what that means, I mistakenly discussed https in my first question)
Thanks for any suggestions in advance, and I humbly admit to be a rank newbie at Exchange.

Answer : Email security between my Exchange Server and a third party Web mail domain

why not just host the mail for 1234.com on your exchange server?