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.