A copy of a incoming email can be sent to a second email address by adding a cc command to the "Custom Rules" field of the mailbox in ISPConfig.
ISPConfig supports sieve as well as maildrop filter rules. The rule language depends on the LDA / POP3 / IMAP-Daemon that you use on your server. If you use courier, then the filter language is maildrop. When dovecot is installed, then the filter language is ieve.
Example in maildrop syntax:
To send a copy to the email address firstname.lastname@example.org, add the following code at the beginning of the custom rules field.
Example in sieve syntax: