I see strange things in my zimbra log file.

NOQUEUE: reject: RCPT from unknown[]: 550 5.1.1 <*some unknown address@*our domain*>: Recipient address rejected: *our domain*; from=<permissively@gmail.com> to=<*some unknown address@*our domain*> proto=ESMTP helo=<190-178-213-12.speedy.com.ar>

There are unknown addresses who trying to send emails to unknown users in my domain.. And that happens every SECOND....

Also suddenly a user in my domain received 912 undelivered emails in her inbox.. All messages she didn't sent... So we changed the password, but after a week, it happens again and again and again..

Is there a bug in my mailserver? What can I do