RECEIVERS-REC_TYPE = 'U'.
RECEIVERS-COPY = 'X' . " this will send the receiver as cc
append RECEIVERS .
This is the only way to send email in CC with F.M 'SO_NEW_DOCUMENT_SEND_API1'
You may want to debug the program and check RECEIVERS-BLIND_COPY if somehow set as 'X'
I don't know if I provided any valuable information here , nothing much I can think of except debugging and checking the flag blind_copy