Please moderators, if you delete this 3rd post of mine, would you care to explain why you did it. I'm posting this in the right forum and I have one very nasty problem which may even be related to bug.

Sometimes, not always (and it succeeds in subsequent attempts), message can't be sent and this is what is returned both in Web client and in Zimbra Desktop:

method:	[unknown]
msg:	Invalid address: xyz <address>. com.zimbra.cs.mailbox.MailSender$SafeSendFailedException: MESSAGE_NOT_DELIVERED; chained exception is: com.zimbra.cs.mailclient.smtp.InvalidRecipientException: RCPT failed: Invalid recipient address: 550 Please turn on SMTP Authentication in your mail client.
detail:	soap:Sender
trace:	qtp1356777178-19278:
As far as a I know, there are no SMTP setting neither in web client nor in Zimbra Desktop, so I really don't know what it's all about. One more time, it happens sometimes, but it happens very frequently!

Please help!