I've been using Zimbra since version 3 and have been a great fan.
A couple minor things I've always wondered but never asked.
1. The 'your message has been sent' message has never fit in the little window down the bottom left.
2. The vertical resize bar has never worked for me.
I'm now running 4.5.4 on Ubuntu Dapper. I have tried Safari and Firefox on OSX and I've tried IE6, IE7 and Firefox on Windows and I've tried Konqueror and Firefox in KDE and Gnome environments.
I've attached an image that will make more sense than 1000 of my words