The Business Connector API is unable to run any queries in the Mail module because it is unable to find the BOs within the module.
Therefore, Kurve Graphs & Reports cannot be created from queries where the Mail Module is the primary BO.
The bug only happens if a Mail module BO is the Primary BO of the query.
This was noted as a TRIRIGA bug and resolved in platform 3.8 https://www.ibm.com/support/pages/apar/IJ26454.
If you need to create graphs and reports using the Mail Module as the primary BO, you will need to upgrade your TRIRIGA Platform to 3.8.