Open Mail Relay: Why It Is Considered A Spammer's Dream https://www.duocircle.com/content/mail-relay-smtp/open-mail-relay
An open mail relay is a Simple Mail Transfer Protocol (SMTP) server configured in such a way that it allows anybody on the Internet to send e-mail through it https://en.wikipedia.org/wiki/Open_mail_relay An open relay is generally understood to be a server that allows emails from any sender to any recipient without the need for authentication. This represents a considerable risk because such servers are often used by 3rd parties to send SPAM. https://www.ionos.co.uk/help/server-cloud-infrastructure.... Being a postmaster is a specialised job. Welcome to the real world. Configure your MTA properly or stop it sending and relay through your ISP instead.
