Before looking at the solution, we need to be clear about certain things. Is the Referral system outside your mail client
(outlook, lotus notes for e.g) and exchange server? I hope it is yes.
And do you want to monitor the inbox in your application (Referral System), or your mail client (Outlook for example)?
If the answer is in your application, then you would need to handle it through polling. If it is a windows application, do a poll to the database every 5 minutes or so, to look for a new member, and use your "Mail module" to send "greetings". If it is a web application, do a constant check using a Json call and do as adviced above.
If the answer is that you need to monitor your mail client like Outlook - then you have two choices.
a) Write a mail client Addin (like outlook addin) that constantly checks for mail with a particular subject or body in the inbox
b) And if it is Outlook, you can then use the Interop assemblies to talk to Outlook Inbox object and figure out if there are mails and take appropriate action. You need to use the "http://msdn.microsoft.com/en-us/library/microsoft.office.interop.outlook._items.find.aspx" Method to figure out the mails as per you need.