useful commands and hints..
generate, check and activate aliases:

 makealiases; makealiases -chk; courier flush

mailq displays a list of all messages that have not been delivered yet:

 mailq|less

find relay errors:

 grep "error,relay"  /var/log/mail.log|less

 courieresmtpd: error,relay=::ffff:9x.2x6.7x.1x5,from=<mymail@m<domain.net>,
 to=<friend@otherdomain.net>: 513 Relaying denied.

outbound authentication must be checked within the email client!

(This is the second method if pop-before-smtp fails.)

Please make sure that “Server requires authentication” is enabled in your email client.