Contact the National Customer Service Center at 1-800-375-5283 and you should be able to find out if it was logged in the system. If it does not show up, you can request an SRMT to determine if the response was received and USCIS will send a response in about 30 days. If you have any concerns/hesitations, please consult with an immigration attorney for advice you can rely on.
You should be ok if you have proof of delivery before the deadline. Check case status online through USCIS website to see if it shows that they received your response to the RFE.
If you want to be extra sure, you can request an INFOPASS appointment with the Santa Ana USCIS.
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Samuel Ouya Maina, Esq. 415.391.6612 email@example.com Law Offices of S. Ouya Maina, PC 332 Pine Street, Suite 707 San Francisco, CA 94104