While not required, I would add the following after your existing TLS-related directives:
smtpd_tls_loglevel = 0
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom
This is not mentioned in the tutorial, but should be: remove your hostname from the mydestinations
directive, so that the line reads
mydestination = localhost, localhost.localdomain
Postfix will complain if a given domain is used for both myhostname
Next, ensure that your Mail settings within the ISPConfig interface are correct. Navigate to System -> Server Services -> Mail and ensure that the values for these two menus are correct:
POP3/IMAP Daemon ...Courier...
Mailfilter Syntax (should be Maildrop based on the main.cf that you posted)
Finally, restart Postfix:
service postfix restart
Any change in the behavior?