# Functions
CheckHost fetches SPF records, parses them, and evaluates them to determine whether a particular host is or is not permitted to send mail with a given identity.
# Constants
A "fail" result is an explicit statement that the client is not authorized to use the domain in the given identity.
A "neutral" result means the ADMD has explicitly stated that it is not asserting whether the IP address is authorized.
A result of "none" means either, (a) no syntactically valid DNS domain name was extracted from the SMTP session that could be used as the one to be authorized, or (b) no SPF records were retrieved from the DNS.
A "pass" result is an explicit statement that the client is authorized to inject mail with the given identity.
A "permerror" result means the domain’s published records could not be correctly interpreted.
A "softfail" result is a weak statement by the publishing ADMD that the host is probably not authorized.
A "temperror" result means the SPF verifier encountered a transient (generally DNS) error while performing the check.