I do require authentication and that is why I configured DKIM and DomainKeys to use 587 as well.
Just to reiterate, my system configuration works as expected with all other email clients tested (Windows Live, Outlook, & Squirrelmail). I am now trying to get Roundcube to work as well.
I was tailing the mail.log file when trying to send a message from Roundcube with the SMTP port set to 587. Nothing was logged. I then tailed the syslog file and here is the sanitized result:
Jul 19 18:30:21 server imapd: Connection, ip=[::ffff:192.168.XX.XX]
Jul 19 18:30:21 server imapd: LOGIN, user=user@example.com, ip=[::ffff:192.168.XX.XX], port=[60637], protocol=IMAP
Jul 19 18:30:21 server postfix/smtpd[12406]: connect from server.example.com[192.168.XX.XX]
Jul 19 18:30:21 server postfix/smtpd[12406]: warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
Jul 19 18:30:21 server postfix/smtpd[12406]: warning: server.example.com[192.168.XX.XX]: SASL LOGIN authentication failed: generic failure
Jul 19 18:30:21 server postfix/smtpd[12406]: disconnect from server.example.com[192.168.XX.XX]
Jul 19 18:30:21 server imapd: LOGOUT, user=user@example.com, ip=[::ffff:192.168.XX.XX], headers=0, body=0, rcvd=10, sent=83, time=0