Banning sender who forges domain to send email to same local domain

[supportterabox]

hi everyone.
now, i were using zimbra for my mail server, but i ran into a problem. someone forged my domain to send mail to my users in my mail server. for example, my domain is mydomain.com and having someone forged a user like "strangeuser@mydomain.com" to send a email to my local users like "mylocaluser@mydomain.com". so i think problem is in my mail server. it did not check ip address of sender whether is my server ip address or not ? since it sent messages directly to user's inbox. i would like to know that zimbra has any method to mark spam a mail if it uses my domain but sender's ip address is not my server address ?.
thanks in advance