I too was searching for this answer. I just moved my zencart powered company site from "nightmare-host" to my local ISPconfig server. I use Google Apps for the domain and had all the MX records entered correctly. I could send/receive emails fine as long as it wasn't to my own domain. I would get errors about "no known user here" in the mail.log. The main problem is my store uses the PHP wrapper to send email and I got a similar disappearing email deal so I started digging and found this. I never thought about the "co-domain" part. Falko... you and Till are AWESOME! Now I just need to figure out how to move from ISPconfig2 to 3. I like the simplicity of 2. Just need a manual to read on 3 so I can know what does what.