I would like to propose the following:

Zimbra release network editions in:

5, 10, 15 and 20 user packs catered to the SMB marketplace, purchasable via the website.

Typical content and costs:

$25 per mailbox per year with 1 x support incident.
$35 per mobile per year over the wire connectivity.

This will ensure that Zimbra competes at the same user level as Microsoft's Exchange Server.