27 C.F.R. § 70.25
(a) When the Bureau summons the records of persons defined by 26 U.S.C. 7609(a)(3) as “third-party recordkeepers”, the person about whom information is being gathered must be notified in advance, except when:
(b) Within 3 days of the day on which the summons was served, the notice required by paragraph (a) of this section shall be served upon the person entitled to notice, or mailed by certified or registered mail to the last known address of such person, or, in the absence of a last known address, left with the person summoned. No examination of any records required to be produced under a summons as to which notice is required under paragraph (a) of this section may be made:
(c) If the person about whom information is being gathered has been given notice, that person has the right to institute, until and including the 20th day following the day such notice was served on or mailed, by certified or registered mail, to such notified person, a proceeding to quash the summons. During the time the validity of the summons is being litigated, the statutes of limitation are suspended under 26 U.S.C. 7609(e). Title 26 U.S.C. 7609 does not restrict the authority under 26 U.S.C. 7602 (or under any other provision of law) to examine records and witnesses without serving a summons and without giving notice of an examination.
(26 U.S.C. 7609)
[T.D. ATF-301, 55 FR 47608, Nov. 14, 1990]