I upgraded our Zimbra server from 5.0.4 to 5.0.16 yesterday. Among the problems reported today is that a user using IE 8 under Windows XP cannot send email via the Zimbra mail web interface.
I watched the user connect to Zimbra web interface, and construct an email by forwarding a message in his mailbox. When he clicked the "Send" button, it went gray'd out but the message did not get sent. It did not go into his sent items, and did not appear in the Drafts either.
Switching the browser to Firefox fixed the issue.
# rpm -qa | grep zimbra