Just another solution : You may also use fetchmail or getmail to fetching the mail from Google Apps and then deliver it to each mailbox in Zimbra. Make sure you use SSL mode to fetch an email from Google Apps and used
username@domain.tld as your user name.
Pros : all setting on one configuration file. You do not need to add an external account for each mailbox
cons : User and password saved on plain text, but the file itself has a 710 permission mode.
Actually, I'm currently using fetchmail+Google Apps for some SME clients.
