The patch described in this thread is for pop3/imap only if I remember correctly and not for smtp. So you will have to use the normal username for smtp logins.
There is a similar patch available for dovecot instead of courier which might work for smtp as well as postfux can authenticate against dovecot. Or you might have to write a new sasl configuration for your setup that uses a different authentication source then the Linux passwd file.
Using the email address for login in ispconfig 2 is always a hack as ispconfig 2 is based on linux users which may not contain a sign, so if you want to use email addresses for mailusers, then use ispconfig 3 which uses virtual suers and not ispconfig 2.