Last year's taxes not received & Line #26

Today is tax filing day for those using an extension.

I used my refund from last year and applied it to this year's taxes which I just filed today.

(e-filed them with an extension)

The problem: I had to send my taxes by snail mail last year and IRS site says they haven't received the previous year taxes.

They were sent. I have USPS proof of delivery.

So: will IRS claim I have an underpayment since they are processing this year's e-filed taxes more quickly and might not see the previous year's taxes in time?

I already tried calling the IRS.  They aren't taking phone calls today. 

I know I would have the proof after the fact, but don't want to trigger an inquiry in the first place. 

Any thoughts?

Thanks you.