SMG Spam email

Hi guys,

In my environment now, we got one spam email outbreak. the worst thing about it is only the top management received it.

its a blank email, no body no attachment just a subject. the sender address is using gmail domain and the username always change. 

SMG doesnt detect this email as apa spam and deliver it normally.

how to stop this issue? or how i want to trace the sender so i cant blacklist or detect who is sending it.

thanks