If you're trying to make a "real world model" of things, then you would have a global PostOffice object, which would contain many Postmen.
The Doctor could be part of a DoctorOffice, or a Hospital
The Doctor would have a member variable pointing to HIS Postman
Doctor.Postman = aPostmanObject;
He would then let the postman do the work.
Doctor.Postman.SendMail();
Assuming you made "SendMail" a public method of Postman (which it would be, as he is doing this as a public service)