MX records are just another type of record along with A records or CNAMEs that you've setup for your website.
If you have moved your name servers from fasthosts to Zerigo then you must also add your MX records to Zerigo too.
In order to add a record to Zerigo, open the addon using $ heroku addons open zerigo
. Then you should click configure to get to the Zerigo panel. From here make sure you're on the hosts page and click add. Change type to MX and set the priority and data to match each entry in the list of Google MX server. Repeating for each Google server.
The list of Google mail server is here.