5039 - Unable to select mail list mode. Received values for both WB_ToField and WB_BCCField. Please remove one!

This error is generated when user tries to execute Mail List command (WB_Command=L) with values set both for WB_ToField and WB_BCCField. When executing L command WhizBase checks these fields to determine what kind of list to generate - if both fields have value WhizBase cannot decide what to do (there is no default method).