An internal messaging system is usually an Intranet or Extranet based web application. This means that it can be limited to who can use it and where they can use it from.
The bespoke messaging systems that we at Visualsoft develop allow staff within companies to contact each other through a secure and web based system.
As the system is bespoke the solution is written to match your exact need. We would welcome discussions with any company looking to incorporate an internal messaging solution into their site or internet/extranet.