I would like to create a live visualization of all the mail communication happening within one "Google Apps" domain used in my organization.
Is there any API which I can use to get a live callbacks, or at least some API which I can pull periodically in order to get headers of all the emails sent within the organization?
Use the Email audit API:
http://code.google.com/googleapps/domain/audit/docs/1.0/audit_developers_guide_protocol.html