Sender Policy Framework (SPF)

wikipedia:Sender Policy Framework (2014)

• https://mxtoolbox.com/SuperTool.aspx

+ for a PASS result. This can be omitted; e.g., +mx is the same as mx.
? for a NEUTRAL result interpreted like NONE (no policy).
~ (tilde) for SOFTFAIL, a debugging aid between NEUTRAL and FAIL. Typically, messages that return a SOFTFAIL are accepted but tagged.
- (minus) for FAIL, the mail should be rejected (see below).

Related

• dig -t TXT

See also

• Mail, SPF, DMARC, MX, DKIM, ARC, Email Authentication, Email spoofing, Sender ID
• Mail, SMTP, submission, SMTPS, POP, IMAP, StartTLS, Exim, Postfix, IRedMail, Fail2ban, Dovecot, Roundcube, DKIM, SPF, DMARC, MX, ssmtp, mailx