unknown addresses in my zimbra log
I see strange things in my zimbra log file.
NOQUEUE: reject: RCPT from unknown[188.8.131.52]: 550 5.1.1 <*some unknown address@*our domain*>: Recipient address rejected: *our domain*; from=<firstname.lastname@example.org> 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