From the perspective of the user's outlook profile finding the mailbox, yes, just creating a CNAME record of one.domain.com and pointing it to two.domain.com will resolve your issue for the outlook clients. However, you will need to ensure that if two.domain.com will be the new smtp server exchanging mail with other smtp hosts, then you will want to update the MX record(s) as well. Mail is delivered between mail systems using the MX record. An Outlook client uses a standard DNS host record to located a mail server.