What happens if my correspondence with a Certificate of Mailing is not received by the USPTO?

If correspondence with a Certificate of Mailing or Transmission is not received by the USPTO after a reasonable amount of time, you can take steps to notify the Office and provide evidence of the previous timely filing. According to 37 CFR 1.8(b):

In the event that correspondence is considered timely filed by being mailed or transmitted in accordance with paragraph (a) of this section, but not received in the U.S. Patent and Trademark Office after a reasonable amount of time has elapsed from the time of mailing or transmitting of the correspondence […] the correspondence will be considered timely if the party who forwarded such correspondence: (1) Informs the Office of the previous mailing or transmission of the correspondence promptly after becoming aware that the Office has no evidence of receipt of the correspondence; (2) Supplies an additional copy of the previously mailed or transmitted correspondence and certificate; and (3) Includes a statement that attests on a personal knowledge basis or to the satisfaction of the Director to the previous timely mailing, transmission or submission.

It’s recommended to check the private Patent Application Information Retrieval (PAIR) System first to see if the correspondence has been entered into the application file before notifying the Office.