Problems with Incoming mail appendage to conversations

Hi,

I am encountering problems with some incoming mail being appended to conversations within a request.

I have successfully enabled the incoming mail function and been able to integrate some replies from requesters into the appropriate request conversations automatically, however from some requesters (notably users from our corporate addresses!) SD+ fails to recognize that there is a request associated with the reply and proceeds to open a new request based on the email.

My question is how does SD+ handle incoming mails and how does it associate them with the correct request? Is this all based on the SMTP headers of the mail? would it affect SD+ ability to correctly process the replies if the mail was first coming through a mail marshal gateway release system?

This as you can imagine is very frustrating and creates additional, unwanted, requests! This for us is a great feature of SD+ that we would like to use to it's fullist.

Please Help!!!
James