Mobilesync through just Zimbra Webmail?

Hi,

Many of my clients have older versions of Outlook (2010, 2007, even 2003). As Zextras Mobile won't work with these old versions, I am wondering if I could persuade these clients to switch to Zimbra Open Source webmail client. But, will full mobile sync of Zextras Mobile work when using just the Zimbra Webmail client? Or, does it require Outlook 2013?

If it's possible, I could probably convince these clients to shift to Webmail after importing their Outlook .pst file into Webmail.

Thanks in advance.