All mail server administrators will have encountered the following problem: spammers send out spam and they spoof the mails so that the sender appears to be a user in the domain you are managing. Unfortunately there is not a lot that can be done to prevent spammers from sending such these spam mail. There is however a way in which receiving mail servers can check whether the mail was send from your mail server or whether is was a spammer who used a mail server that is not authorized to send mails from your domain. This can be accomplished with SPF (Sender Policy Framework).