If you are using BCS class, call the method below:
DATA lo_send_request TYPE REF TO cl_bcs. (You already have declared this with another name)
DATA lo_sender TYPE REF TO if_sender_bcs.
*--- set the e-mail address of the sender:
IF NOT lv_sender_email_addr IS INITIAL.
lo_sender = cl_cam_address_bcs=>create_internet_address(
i_address_string = lv_sender_email_addr).
ELSE.
lo_sender = cl_sapuser_bcs=>create( sy-uname ).
ENDIF.
lo_send_request->set_sender( lo_sender ).
Regards,
Felipe