you can create an object group with those allowed ip/networks then create an access-list rule that allows SMTP only from this object group
you do not need to do this on your Exchange 07 server because as long as your ASA is doing its job, it'll never see SMTP traffic from any other ip/networks besides the ones in this object group
http://www9.cisco.com/en/US/docs/security/asa/asa72/asdm52/user/guide/aclrules.html