Match Email Messages Between Mailboxes

Question

If I have access to two mailboxes (User A and User B).  How would I determine if two email messages (one in User A's inbox, and one in User B's mailbox) represent the same email message?  Would they have the same InternetMessageId?  
I am building an application that will merge emails from several different users inboxes into a single repository.  I want to avoid creating duplicate entries for the same email messages where one user sent an email to another user in the group.
When merging two different users email folders how do I identify two EmailMessages that represent the same email sent between the two users?

Answer 1

If its two separate mailboxes I would look at the Internet Message Id see
http://msdn.microsoft.com/en-us/library/office/cc839521%28v=office.15%29.aspx PidTagInternetMessageId. That should work in most case it not guaranteed though so I would expect you will always have some duplicates but its probably the closest you'll
get.
You could also use your own transport agent eg if you catch your messages in an OnSubmission before any categorization takes place and add your own unique X-header in the Message you could then use that value on any messages that are
delivered to ensure uniqueness.
Cheers
Glen