We've had several reports from Android users claiming that the Zimbra Web server doesn't recognize the Android as a mobile device and kicks them to the full Web page by default. Moreso, if they explicitly go to the mobile page and click around they sometimes get booted back to the full-featured Web page anyhow.

Is there any way to add to the list of devices that are treated as mobile devices by default?

The issue seems to be very similar to the one that drupal ran into:

Android 2.1-2.2 not a Mobile browser | drupal.org