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We will soon be forced to use PKI Encryption for our communications with our largest customer (also using Zimbra). I don't see any mention of PKI in the forums, to documentation, or the wiki, so I thought I'd post it here - we need it!
Welcome to the forums. The best thing to do is search bugzilla for a request for enhancement, if one doesn't exist, go ahead and create one, and dont' forget to vote for it.
Bugzilla
Wiki
Source
PKI Encryption 
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