I got form 3531 with checkbox #1 checked, claiming that my return doesn't have a "valid original signature."

However, my spouse and I both signed on the signature lines. The original (not a photocopy) 1040 was returned to us with the 3531, and I've confirmed both signatures are present, on the right lines, and dated.

I've been trying to call the IRS directly but haven't gotten through so far. If anyone has any insights as to what they might expect me to do (should I just mail it back as-is?), it would be appreciated.

Despite this demand letter, we've already received our refund.