I am not familiar with any option where emails are allowed cover pages, atleast from a printer configuration standpoint. Cover pages are normally only included for printing jobs from the print driver or print server area.
If you believe there is a setting on the printer that is causing this behavior then perhaps you should reset the printer and reconfigure with these steps from HP:
Scan to folder - http://h20566.www2.hp.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-c03233046&sp4ts.oid=5245583
Scan to email - http://h20566.www2.hp.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-c03231533&sp4ts.oid=5245583
If the coverpages you seek to disable are not resting on your printer then perhaps you have some other email system that is interferring and would need reconfiguration.