11 CFR 100.27: Mass mailing (52 U.S.C. 30101(23)).

Mass mailing means a mailing by United States mail or facsimile of more than 500 pieces of mail matter of an identical or substantially similar nature within any 30-day period. A mass mailing does not include electronic mail or Internet communications. For purposes of this section, substantially similar includes communications that include substantially the same template or language, but vary in non-material respects such as communications customized by the recipient's name, occupation, or geographic location.