SMTP authentication for local emails

[extremal]

Hi All,
I have a problem and the problem is that my emails are being spammed with local accounts. In my current installation I have 5 domains on my server and when you send email from one user to another user of the same domain SMTP authentication is not required. I think because of that I am receiving a lot of spam mails.
How can I enable autentication for my SMTP even for local accounts to stop that spamming and do not affect normal email usage? How can I validate senders mail addresses and stop spooffing?

thanks,

[uxbod]

Have you had a read through ? Wiki :: Improving Anti-Spam System

What do you have for

su - zimbra
zmprov gcf zimbraMtaRestriction

[extremal]

Thanks for your reply,
No actually I haven't read it.
here's output :
[root@mail ~]# su - zimbra
[zimbra@mail ~]$ zmprov gcf zimbraMtaRestriction
zimbraMtaRestriction: reject_invalid_hostname
zimbraMtaRestriction: reject_non_fqdn_sender

[uxbod]

It would also be worth adding in some RBLs. Personally I have the following config

[zimbra@office ~]$ zmprov gacf | grep -i mtarestriction
zimbraMtaRestriction: reject_non_fqdn_sender
zimbraMtaRestriction: reject_invalid_hostname
zimbraMtaRestriction: reject_rbl_client b.barracudacentral.org
zimbraMtaRestriction: reject_rbl_client zen.spamhaus.org
zimbraMtaRestriction: reject_rbl_client dnsbl.sorbs.net

for the barracuda RBL you would need to sign up for free at Barracuda Central.