This is an odd one and searching around I've come up empty handed. I'm fully sure it's a bug, so I'll wait until after I've gotten a few responses here to file a bug report.
Zimbra 7.1.4 / Ubuntu 10.04 LTS Server x64 using Active Directory Auth from a windows 2008R2 server.
Basically I have 1 user out of 225+ who has this issue. When he goes into his mailbox (via any of these browsers: IE8, FF3, Crome) he just can't send email. It opens the compose window and he can create and even save drafts of the email. But when he hits the send button, it just sits there, eventually times out and sometimes gives the "server unavailable" error.
Occasioanlly the message will get out anyway, but its not consistent. Occasionally saving it to drafts and then attempting to send it from there works, but its not consistent either.
I've tried using his account on different computers - same result.
I've logged in with multiple accounts on the computers he's using - does not reproduce the issue.
I've tried renaming this account and recreating a new account with his username - same result.
I've tried deleting and recreating his AD account - same result.
No errors in any logs associated with his username.
No suspicious log entries associated with his username
No errors in logs that seem to be related in any way.
The only thing I haven't tried yet is a full server reboot, which I plan to do this weekend.