We also required a catchall without mailbox solution for Dovecot/Sieve on ISPconfig 3. I found that these steps work perfectly.
1. Create a new mailbox for the domain (e.g.: email@example.com
2. Create a catchall for the domain that points to new mailbox.
3. Edit mailbox and go to the "Custom Rules" tab. Enter these two rules:
If I use these rules, an email sent to "firstname.lastname@example.org" is redirected to "email@example.com" and discarded. No messages are saved in the catchall mailbox! You MUST include the "stop;" line or Sieve will NOT discard the message after forwarding.
Suppose your catchall mailbox is "firstname.lastname@example.org". Login to the mail server that hosts the mailbox and type the following to view the entire Sieve filter for this mailbox.
Contents of the Sieve filter file are as follows.
require ["fileinto", "regex", "date", "relational", "vacation"];
Notice the "keep;" line added by ISPconfig 3 at the end of the file? If you do not specify "stop;" to terminate the Sieve script, Dovecot will execute the "keep;" command and place a copy of the message in your catchall mailbox.