I am having problems with image attachments as well.
If someone attemps to view an image that was embedded in a forwarded email, the user has issue with the image being linked to my Zimbra server. So they'll get a prompt to accept my cert, and then the image fails to load. The result is them having to manually launch the attachments and the contextual placement of the image is lost.
For clarity... I received an email with embedded images, I forwarded it to someone else, the email they receive from me attempts to pull the embedded images from my zimbra server.
I've yet to check bugzilla, I'll do that next. I figured I'd let you know you're not alone in image attachment hell
It seems to be happening even on a Reply as well, not just a forward.
Here is the code that was in a message which behaved the way I described but on a Reply. This was in the message that someone received from me.
<p class="MsoNormal"><img width="290" height="349" id="Picture_x0020_1" dfsrc="cid:image003.jpg@01C8D206.6C3887E0" src="https://zimbra.domain.com/service/home/~/?auth=co&id=21631&part=1.2.3&t=1213903835859"></p>