If enabled, the SPF record will apply only to the Virtualmin server itself, and all hosts specified in the previous option. This is the preferred method of SPF usage, as it strictly limits the hosts which can send mail claiming to be from this domain. However, if users must be able to send from arbitrary mail servers (such as those of their ISP), disabling this option may be necessary. It is, however, likely to lead to unfortunate side-effects in the future, as mail servers begin blocking hosts with no SPF record or particularly lax SPF records. Read more about SPF at the Sender Policy Framework homepage.