Hello, i'm trying to add the reject_unknown_sender_domain to my main.cf under smtpd_sender_restrictions . Now the settings look like this: The "access" file is empty. Nethertheless, even though postfix shouldn't send e-mails with unknown senders like "[email protected] I tried it with on the postfix server and the mail went out and arrived at the mail account. But that's not what i want. I don't want postfix to pass mails from internal clients, where the domain is unknown. This makes postfix quite open in my opinion. Does anyone know, how to restrict this?