26 C.F.R. § 20.2205-1
If any portion of the tax is paid by or collected out of that part of the estate passing to, or in the possession of, any person other than the duly qualified executor or administrator, that person may be entitled to reimbursement, either out of the undistributed estate or by contribution from other beneficiaries whose shares or interests in the estate would have been reduced had the tax been paid before distribution of the estate, or whose shares or interests are subject either to an equal or prior liability for the payment of taxes, debts, or other charges against the estate. For specific provisions giving the executor the right to reimbursement from life insurance beneficiaries and from recipients of property over which the decedent had a power of appointment, see sections 2206 and 2207. These provisions, however, are not designed to curtail the right of the district director to collect the tax from any person, or out of any property, liable for its payment. The district director cannot be required to apportion the tax among the persons liable nor to enforce any right of reimbursement or contribution.
26 C.F.R. § 20.2205-1