Mail check: SPF, DKIM, DMARC, MX

The check only reads DNS and sends nothing. The SPF mechanisms a, mx, exists and ptr count toward the lookup limit, but their targets are not checked. If no DKIM selector is given, we try common ones (default, google, selector1, selector2, k1, k2, mail, dkim, s1, s2, mandrill, smtp). A subdomain may have no DMARC record of its own: check the organizational domain instead (for example example.com rather than mail.example.com).