Del. Code tit. 12, § 2352

At any time before the expiration of 18 months after the qualification before any Register of Wills in this State of any executor of the will or administrator of the estate of any nondomiciliary decedent, such executor or administrator shall file with such Register proof that all death taxes, together with interest or penalties thereon, which are due to the domiciliary state of such decedent, or to any political subdivision thereof, have been paid or secured, or that no such taxes, interest or penalties are due, as the case may be, unless it appears that letters testamentary or of administration have been issued on the estate of such decedent in the domiciliary state. The proof may be in the form of a certificate issued by the official or body charged with the administration of the death tax laws of the domiciliary state.