Thank you for your correspondence received May 10, 2006.
We haven't resolved this matter because we haven't completed all the research necessary for a complete response. We will contact you again within 45 days to let you know what action we are taking. You don't need to do anything further now...
That's all well and good, except I've not corresponded with the IRS. I filed my 1040 and my 1040ES electronically in April, and just mailed off the 2nd quarter estimated check yesterday. This is supposed to relate to the 2005 1040.
The letter gave a phone number to call, so I did. After negotiating the phone tree (there was no option for "received mysterious letter"), I finally got put n the queue and eventually got to speak to a Real Person. First she had to confirm that I am me, at my address, have my birthday, and filed electronically this year. She was able to tell me that my estimated payment had been received after I told her the amount (some miracle of the USPS? or maybe she saw the one from April), but knew nothing more about the letter than what it said: it's pending, and relates to some correspondence I supposedly sent them.
'Tis a puzzlement. I shall wait for the further contact in hopes it will tell me more.