Sometimes, a report gives an error that there are too many contacts to return the results and you must export the results. In some cases, such as in a Single Mailing Report, having to export means you do not get all the data that you want.
Contact the client support and request your Org Settings > Reporting Settings to be changed from Max # of mailing contacts to allow viewing detailed tracking: from 5000 to a higher number. If a email went to 100,000 contacts, this number must be larger than 100,000.
Note: If you set the number too high, application time outs can occur.