SMTP Authentication for Local Domain

[Lubomir]

Hi everyone,
Does anyone solve successfully this behavior?

When some (e.g. spammer) sends an email with FROM: anything@mydomain.com TO: existing_user@mydomain.com SMTP server in Zentyal doesn't require authorization and in a log it's marked as internal. A session is established from an interface marked as External.

I need to achieve the postfix require authorization for all connections except the situation FROM some@one.com TO existing_user@mydomain.com In this situation is of course no authentication required :-)

Thank you for hints.

Regards,
       Lubomir