Hi
Check in below link..
Sending email via CL_BCS - Supressing read receipt request
lro_send_request->SET_STATUS_ATTRIBUTES( I_REQUESTED_STATUS = 'E' ).
If you set the I_REQUESTED_STATUS 'E' you will see the sent mail in the transaction SOST.
If you set the I_REQUESTED_STATUS 'N' you won't even see the mail in the SOST.
Regards,
Murthy