Some really broken/misconfigured mailer software may still accept the 'foo@bar@qux' syntax (route mail for user foo@bar via qux, or the other way round - I forg

Some really broken/misconfigured mailer software may still accept the 'foo@bar@qux' syntax (route mail for user foo@bar via qux, or the other way round - I forgot which, since no sane system has implemented this since the word 'spam' came to mean bad e-mail.) So there may, theoretically, be some value in checking for the presence of exactly one @.