If the tax or any part thereof is paid or collected out of that part of the estate passing to or in possession of, any person other than the executor in his capacity as such, such person shall be entitled to a reimbursement out of any part of the estate still undistributed or by a just and equitable contribution by the person whose interest in the estate of the decedent would have been reduced if the tax had been paid before the distribution of the estate or whose interest in the estate is subject to an equal or prior liability for the payment of tax, debts, or other charges against the estate, it being the purpose and intent of this section that so far as is practicable and unless otherwise directed by the will of the decedent, the tax shall be paid out of the estate before its distribution; but the commissioner shall not be charged with enforcing contribution from any person or persons.