Zimbra is both a commercial and community based email platform. It is not solely a webmail system, but a complete replacement with an MTA based on Postfix, and a POP/IMAP server which integrates with clients like Thunderbird and Outlook to provide address book services. Zimbra is a complete communications platform, but as such would require the current Plusnet platform to be replaced to get the benefits that it can offer. It is possible to run Zimbra on top of Plusnet's platform, but in this case the email would have to be stored twice, and the two would not be consistent without constant synchronization and the race conditions this introduces. Its time to implement would not be quick, but its feature set is impressive. Plusnet have already had discussions with Zimbra, who is owned by Yahoo, and except for the limitations of having to run the platform on Linux, rather than having the freedom to select Solaris or Linux, it is seen as a viable solution. However, a platform based on this would not be seen in the near future simply based on the development time required to integrate the system.
Geoff, I assume the link you where giving about is just http://www.zimbra.com/. ; I had a look at Zimbra a while ago and was very impressed with its feature set and it's look and feel. The demo on their website is well worth a look. Phil