SPF, or Sender Policy Framework, is a means to help reduce spam by enforcing some restrictions on what hosts can send mail claiming to be from what domain. Because the vast majority of spam originates from faked domain names, it is believed by many that preventing forged domain names in spam will reduce the overall volume of spam. If enabled, Virtualmin will automatically create an SPF record in your DNS server. This record can either contain specific hosts that are allowed to send mail using the domain name in question (with the server itself being the first such host, but any number of hosts can be listed), or a generic catch-all, which means any host can send mail claiming to be from this domain. The second option effectively nullifies the purpose of SPF, but is provided to make it easier for users to send mail "from" their own domain through various ISP mail servers.