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.
The costoxiphoid ligaments and the side of the xiphoid process are often attached with some of the fibers of the muscle. The rectus sheath, formed by the aponeuroses of the transversus abdominis and oblique muscles, encloses the flat muscle and holds it in position.