I have ispconfig with many sites and mail accounts. I have problem with many (60k) mails in mail queue. This mails comes from different mail addresses like this email@example.com. existsing-domain.tld have few real mail boxes. Maybe credentials to one of his mailboxes have been stolen. I have cleared mail queue, but I need to prevent this from happening again. 1. How can I reject mails send from non existing users. I tried in /etc/postfix/main.cf Code: smtpd_sender_restrictions = check_sender_access mysql:/etc/postfix/mysql-virtual_sender.cf, permit_mynetworks, reject_non_fqdn_sender, reject_unknown_sender_domain, permit but it does not seems to work. 2. How can I prevent sending mails from different domain? As I tested I found that if I log in mail server with firstname.lastname@example.org I can send email from email@example.com. 3. How can I restrict users from sending more than X mails an hour, and every mail after that to be rejected and not get in to the queue altogether? I suppose those are common issues, but I couldn't find guide which address them.