I've noticed a strange behavior of Zimbra lately: when contacting a server which has graylisting enabled the mails get bounced back instead of retried later!

Code:
Sep  5 09:50:34 webmail postfix/smtp[5776]: 894D6CAC02E: host mx.destdomain.it[] said: 450 4.2.0 <gianni.fioretta@destdomain.it>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/destdomain.it.html (in reply to RCPT TO command)
Sep  5 09:50:35 webmail postfix/smtp[5776]: 894D6CAC02E: to=<gianni.fioretta@destdomain.it>, relay=my.zimbra.it[]:25, delay=0.7, delays=0/0/0.59/0.11, dsn=5.0.0, status=bounced (host my.zimbra.it[] said: 530 <zimbra@zimbradomain.it>, Authentication does not match this account (in reply to MAIL FROM command))
why the mail isn't kept on the postfix queue until the next retry, instead of bounced back to the sender??
thanks