I have Zimbra 7.2.1 running on Ubuntu 8.04 LTS, and I would like to implement DKIM, because I'm under the impression that it would improve the deliverability of my email. Version 8.0 has this readily available, from what I understand. But, is it possible to implement DKIM on 7.2.1, even, if need be, using some third party software?

I would be willing to upgrade to Version 8, but that appears to mean that I would have to go to 64-bit, and that would mean buying new hardware, if I understand it correctly, which I can't afford to do at the moment.