How to stop "open relay" function on smtp server?

@darion 

There are several ways to stop the "open relay" function on an SMTP server:

1. Implement sender authentication: Require users to authenticate themselves before sending emails through the server. This can be done by using SMTP authentication mechanisms such as SMTP-AUTH.
2. Restrict the domains that can be relayed: Configure the server to only allow relaying for specific domains or IP addresses, rather than allowing any domain to relay through the server.
3. Implement access controls: Restrict access to the SMTP server based on IP address or domain to prevent unauthorized users from using the server as a relay.
4. Enable relay restrictions: Configure the server to only allow relaying for authenticated users or users within a specific IP range.
5. Keep the server updated: Ensure that the server software is regularly updated to patch any security vulnerabilities that could be exploited to turn the server into an open relay.

By implementing these measures, you can prevent your SMTP server from being used as an open relay and reduce the risk of unauthorized users sending spam or malicious emails through your server.