Add warning if sp tag is explicitly set for a subdomain

This isn't an error, but according to RFC7489, "'sp' will be ignored
for DMARC records published on subdomains of Organizational Domains
due to the effect of the DMARC policy discovery mechanism."  Therefore
we want to warn users that the sp tag they have explicitly set is
probably not doing what they intended.

See here for more details:
https://tools.ietf.org/html/rfc7489#section-6.3

Author: jsf9k

trustymail/trustymail.py

@@ -493,6 +493,11 @@ def dmarc_scan(resolver, domain):
                 value = options.split('=')[1].strip()
                 tag_dict[tag] = value
 
+            # Before we set sp=p if it is not explicitly contained in
+            # the DMARC record, log a warning if it is explicitly set
+            # for a subdomain of an organizational domain.
+            if 'sp' in tag_dict and not domain.is_base_domain:
+                handle_error('[DMARC]', domain, 'Warning: The sp tag will be ignored for DMARC records published on subdomains. See here for details:  https://tools.ietf.org/html/rfc7489#section-6.3.', syntax_error=False)
             if 'p' not in tag_dict:
                 msg = 'Record missing required policy (p) tag'
                 handle_syntax_error('[DMARC]', domain, '{0}'.format(msg))