I upgraded from Zimbra 5.0.18 to 6.0.0 (Ubuntu 8.04 32 Bit).
The 5.0.18 Zimbra configuration was configured last November and worked fine. As I have a dynamic DNS, I use the DynDNS mailhop service. See Outgoing SMTP Authentication - Zimbra :: Wiki
Since I upgraded to 6.0.0, sending e-mail is no more possible. /var/log/mail.info shows:
The sender receives an error message with the same information. What has changed in email@example.com,relay=outbound.mailhop.org[99.999.999.999]:2525, delay=1.1, delays=0.03/0.01/0.96/0.11, dsn=5.0.0, status=bounced (host outbound.mailhop.org[99.999.999.999] said: 550 You must authenticate to use MailHop Outbound (in reply to MAIL FROM command))
I noticed that the postmap command behaves differently: