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?