First off - please make sure to list your Zimbra version in your profile. You can do this by going to:
As to the specific issue, there are two for this. If you're on the Network Edition, the View Mail button is still there, but the button is all the way on the right.
If you're on the open source edition, the Zimlet was removed. You can find more including a resolution at:
State University of New York at New Paltz