I'm seeing a trend here regarding Zimbra with IE8 and external images. it won't allow them to reply or forward. I tested it and verified the problem myself, as well having several users report it as well.

IE8 and a message with external images are the keys. I'm using NE 5.0.16 on RH 5.3 Ent on the server side as well.

Try replying to or forwarding a message with external images and all I get are buttons that grey out and a message from IE8 regarding sending data from outside web pages. It then procedes to sit there doing nothing.