handle SPF_RESULT_TEMPERROR result

[jcbf]

Should return a temp error according to RFC 7208
https://tools.ietf.org/html/rfc7208#section-8.6

8.6. Temperror

A "temperror" result means the SPF verifier encountered a transient
(generally DNS) error while performing the check. Checking software
can choose to accept or temporarily reject the message. If the
message is rejected during the SMTP transaction for this reason, the
software SHOULD use an SMTP reply code of 451 and, if supported, the
4.4.3 enhanced status code (see Section 3.5 of [RFC3463]). These
errors can be caused by problems in either the sender's or receiver's
DNS software. See Appendix G.4 for considerations on developing
local policy.

A configuration keyword may be used to specify the behaviour ( accept or temp reject ).