Courier mail server esmtp daemon
The actual Courier ESMTP daemon (mail transfer agent) is called courieresmtpd. Normally, courieresmtpd is run from couriertcpd, which is in turn run from esmtpd start or a similar command. You should not have to run courieresmtpd manually.
couriertcpd(8), esmtpd(8).