Respond back telling them that you mailed the extension timely and send a copy of the certification ... chances are real good your extension is buried in the millions of envelopes that have not yet been opened and posted due to the pandemic.
And next time Efile an extension early ... do it by 3/1 if you think you will not file timely at that point ... do NOT wait until the last minute. Then if you are ready to file before the extension period is over you can ... there is no rule saying you must file on the last day.
