In CRM 4.0, after a user sends out an email and the recipient replies to the email, the incoming email is tracked to CRM and the tracking token is shown in subject line.
However the tracking token value is missing from Emailbase table in SQL for the replied email record, and that reflects an empty value in TrackingToken column in Advanced Find as well.
This can happen if the registry key DisableSmartMatching is missing.
Open registry editor (regedit.exe) and locate the hive HKEY_LOCAL_MACHINE\Software\Microsoft\MSCRM
Create a new DWORD of ‘DisableSmartMatching’ with a value = 1
Dynamics CRM Team