Help & FAQ - Apps
Google Apps: I cannot access any subdomains, e.g. mail.domainname.com
If you want to access Google Apps email, calendar, docs at the respective sub-domains, e.g. mail.yourdomain.com or calendar.google.com, you need to add this in your Google Apps dashboard:
https://www.google.com/a/cpanel/REPLACE_WITH_YOUR_DOMAIN.COM/CustomUrl