Visit your servers > mta tab > you didn't happen to change the 'Web mail MTA Hostname:' by chance did you? (While we're at it, what's your 'enable dns lookups' checkbox set to?)
An error from your web-client (pic maybe?), mailbox.log, or zmmailboxd.out (in /opt/zimbra/log/) is gonna be needed sometime - otherwise there's a few hundred things it could be... |