Me. Rev. Stat. tit. 36, § 947

In an action to foreclose a tax lien mortgage under section 944, 945 or 946, the proceedings from and including the assessment of the tax upon which such tax lien mortgage is based to and including the time of filing the complaint in such action need not be set forth in the complaint, pleaded or proved and are presumed to be valid. A defendant alleging any invalidity or defect in such proceedings shall specify in the defendant's answer such invalidity or defect and establish the invalidity or defect.