Processing the check is IRS's responsibility. If they accepted your check, but do not process it, it is their fault. Once they accept your check, your end of the deal has ended. A while back, I got a notice that I did not pay my state tax.
1 The xiphoid process (Latin: processes xiphoideus) is a small triangular cartilaginous extension of the breastbone.
palatine process. A thick process that projects horizontally mediad from the medial aspect of the maxilla. An outgrowth on the ventromedial aspect of the maxillary process that develops into the definite palate.