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*Use SASL or some other means of checking that the person trying to use the SMTP server has a valid user on the system. This way only your users will be able to use your server to relay email.
=== Sender Policy Framework (SPF ) ===
'''Sender Policy Framework (SPF )''' uses DNS to publish a list of server IP addresses that are allowed to send email for your domain. In this way, a receiving server can check whether the sending server is authorized (i.e. message is likely not spam) or not (i.e. message is probably spam).
SPF is a pretty cool system, but it's not perfect. It works very well for single servers but if you send mail for your domain from multiple servers (and perhaps a varying number of them), you are required to use wild-cards which reduce the effectiveness of this system.
