Hi Matthev,
If we send the subject i_subject = p_sub it will come in the SOST but we can set the subject more than 50 lines.
In this following class method they are checking that subject is initial than only subject will set otherwise it wont set the subject.
CALL METHOD lo_send_request->set_message_subject
EXPORTING
ip_subject = t_sub.
Regards,
Ravi Shankar.L