+---------+ +---------+ +---------+ | sender1 | | sender2 | | sender3 | +---------+ +---------+ +---------+ | | | +----------+ + +----------+ | | | v v v +-------------+ | sendmail | +-------------+ | | | +----------+ + +----------+ | | | v v v +---------+ +---------+ +---------+ | mailer1 | | mailer2 | | mailer3 | +---------+ +---------+ +---------+