Costales, Bryan; Assmann, Claus; Jansen, George; Shapiro, Gregory Neil (2007). Sendmail. Oreilly Series (4 ed.). O'Reilly Media, Inc. p. 379. ISBN9780596510299. Retrieved 2012-08-02. As an aid in preventing [...] attacks, V8.1 sendmail first offered the smrsh (sendmail restricted shell) program.