I have been doing some testing with URLs becoming active and here are my results:

When I compose an email to myself the links are active before and after.
When Client A composes an email to himself the links are active before but not after...

I did a test by signing up with 2 forum accounts, one I registered my Zimbra email address and one my gmail address, gmail shows the activation links correctly, but Zimbra shows the links as plain text, thus I needed to copy and paste them into a browser for them to work.

Myself and Client A are composing in HTML, viewing in AJAX and have the com_zimbra_url loaded and enabled.

Any other thoughts?