E-mail auto-linking is a feature of Business Contact Manager that automatically links incoming email message with corresponding Business Contacts or Accounts. BCM links messages if the e-mail address in message matches with any business contact or account email address.
If BCM database is private (i.e. it is not share out to other users), all business contacts and accounts are opted-in by default for email auto-linking. If database is shared out to other user's, all business contacts and accounts are opted-out for email auto linking. It's designed this way for privacy reason so that each BCM user can selectively choose the busines contacts or accounts he/she wants to enable for automatic email linking. You can control what contacts/accounts you want to enable for email linking through Business Contact Manager->Manage Email Auto Linking dialog.
Furthermore, In Manage Email Auto Linking dialog, you can select/unselect Outlook folder to control the auto linking behavior. For example, if you don't want to automatically link the emails that you send, you can uncheck "Sent Items" folder. If certain emails get delivered to folder other than "Inbox" (say if you use Outlook rules to move messages), you can control the link behavior for those emails by checking/unchecking that folder.
BCM monitors all Outlook folders (except some special folder such as Junk Email, Outbox, Draft) for new emails. When it notices a new email, and the recipieint email addresses matches the email of business contacts/accounts, BCM will create a communication history item called "Business Activity". BCM reads important information from email (subject, recipieint addresses, and part of the message body) and stores this information in database. BCM also stores a unique identifier that points to original email message. This identifier is basically a pointer to original email.
When user tries to open this linked email, BCM will attempt to find original e-mail message with unique identifier it has stored. If the original email is found, BCM will open that. If original email is not found in that profile, BCM will open the business activity with the partial data that was saved to BCM database.
Since BCM database and Outlook data files (such as PST) are seperate entities, there are few scenarios where user might see business activity instead of original email.
1. If user migrates Business Contact Manager data in a new Outlook profile that doesn't have the Outlook data file that contains original email. This could happen if you are migrating BCM database form previous version. Or user has removed Outlook data file that contained original email such as PST from the profile.
2. If user deletes the original e-mails that are linked to account or business contacts.
3. If user opens a linked email for shared contacts and accounts, and link points to original email in other user's mailbox. For example, User A and User B are sharing BCM database. User A receives an email that gets linked to a business contact or account. When User A opens this linked item through commincation history, the original email will open. But if User B opens the communication history item, Business Activity form will open.
I hope this post helps understand the logic behind Email Auto Linking. Please feel free to comment if you have any questions.