FINRA Rule 3110.09 addresses retention of correspondence and internal communications amongst firms. Each firm must retain all internal communications and correspondence of associated persons related to the firm's investment banking or securities business for the period of time and accessibility requirements specified in SEA Rule 17a-4(b).
Link to Rule: http://finra.complinet.com/en/display/display_main.html?rbid=2403&element_id=11345
Helpful Link: http://finra.complinet.com/en/display/display.html?rbid=2403&element_id=11470