I have an easy fix for this. I had to do this just last week and it works great.
Set "External Mail Server" in Ispconfig
Then you have to add some MX records for your domain. Depending on your domain name provider you may have to turn on "User MX" pretty easy to do. So then login to your domain name provider's panel to get to the MX record panels.
Add the following MX records to the domain:
Then you're set with google apps email servers for the domain while still using the ispconfig servers apache web server.
This method allows google mail servers to take full control of the domain's mail. If you have any questions or need help I'd be happy to walk you through it on the phone or through skype if you have trouble & need help. I've got mad love for ISPConfig peeps & know how it is too urgently need a helping hand.
Hope this helps
P.S. Oh yeah... and if you want full control domains with a panel to easily set host records to point to your ISPConfig server you can get them here for $10.95
I use that domain panel mainly only for friends now because the checkout is directly through the registrar... easy to point to your own server and edit host records, dns records, mx records etc... no advertisements. But it seems you already have a domain and are all set to go with those MX records above. Email khayjake@gmail if you're interested in that... we both check it daily. Take care & good luck!
-Kextra1 & Khayjake