Solution:
The MTC task is responsible for collecting message routing data into the the Domino MailTracker Store (mtstore.nsf) so that messages may be tracked. The mtstore.nsf must have a full text index (FTI) in order for any Message Tracking functionality to work properly (including data collection, message tracking tools and mail usage reports). MTC must be the only task full text indexing mtstore.nsf.
For optimal performance a periodic rebuild of the mtstore.nsf FTI is recommended. A rebuild will ensure that the FTI remains at the best size for full text searching.
A rebuild can be scheduled using a Program document configured to execute a 'tell' command for MTC.
Program Name: server
Command Line: -c "tell mtc reindex"
In Domino 7.0, you can set MTCCompactReIndex in the INI file so that a daily re-index will be executed when MTC does its daily maintenance.
MTCCompactReIndex=1
If you do not wish to re-index daily, you can schedule a Program document for a custom schedule (weekly, etc). It is not recommended that a rebuild be scheduled at a frequency greater than daily.