The tip of the day says mark the envelope with the type of information inside. Example "response for RFE"or " RFE IMBRA" along with your A number and reciept notice number.
I remember seeing a simular suggestion on the USCIS site.
It would be terrible if your rapid response to the IMBRA sat in the wrong pile for a couple of months. I understand USCIS is very responsive if they know it is a RFE
But on the cheery side, maybe they are slow with no applications to process and will open the mail right away. good luck
