on my ZCS I host approx. 100 domains and I notice a lot of messages from <> and <localhost> sent out and being refused by recipent server. For example, this is my DAILY statistics for yesterday:

Grand Totals

6588 received
8341 delivered
0 forwarded
28 deferred (485 deferrals)
52 bounced
480 rejected (5%)
0 reject warnings
0 held
0 discarded (0%)

Host/Domain Summary: Messages Received (top 50)
msg cnt bytes host/domain
-------- ------- -----------
1596 162061k localhost
1137 94897k someproperdomain.com
235 151504k gmail.com

top 50 Senders by message count
3192 from=<>
230 one.user@one-of-domains.com
120 another.user@another-domain.com

And one example from log, one of deferred messages:
0892220C1CD     4759 Sun Nov  7 18:30:03  MAILER-DAEMON
(host alt1.gmail-smtp-in.l.google.com[] said: 450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered. Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, please 450 4.2.1 visit http://mail.google.com/support/bin/answer.py?answer=6592 c22si2926382ana.195 (in reply to RCPT TO command))
(I'd love to see headers of this message, but when trying to go into postfix mail queue directory, I get Access denied, and also when issuing command postcat -q 0892220C1CD as zimbra user, I get Permission denied.)

Is such a behaviour normal?
I mean, so many messages from <> and <localhost> being in DEFERRED queue?