I have a customer who uses a .gif file as the company signature. Each employee has a gif with his own information so that each can have his/her own signature.
When you're creating the signature and hit the "add image" button, a username/password popup appears on the screen. If you enter it, it works ok. If not, you get a broken link on the signature.
The same happens when you go to the briefcase and try to preview the file that is uploaded after you insert the image on the signature.
I discovered that it happens because the zimbra server's hostname is mx.example.com but people use webmail.example.com to access the web gui.
If they access through mx.example.com it works just fine but i'm not much into changing their zimbra hostname...but if that's the only solution
Zimbra version is Release 6.0.4_GA_2038.UBUNTU8_64 UBUNTU8_64 NETWORK edition