Md. Code Ann., Est. & Trusts § 5-304

(a) Unless a timely request for judicial probate has been filed pursuant to subsection (b) of this section, or unless a request has been filed pursuant to § 5–402 of this title within six months of administrative probate, any action taken after administrative probate shall be final and binding as to all interested persons. Except as provided in subsection (b) of this section, a defect in a petition or proceeding relating to administrative probate shall not affect the probate or the grant of letters.

(b) An administrative probate may be set aside and a proceeding for judicial probate instituted if, following a request by an interested person within 18 months of the death of decedent, the court finds that: (1) The proponent of a later offered will, in spite of the exercise of reasonable diligence in efforts to locate any will, was actually unaware of the existence of a will at the time of the prior probate; (2) The notice provided in § 2–210 of this article was not given to such interested person nor did he have actual notice of the petition for probate; or (3) There was fraud, material mistake, or substantial irregularity in the prior probate proceeding.