fix: authenticate_message: Do not set None to spf_result.smtp_mailfrom

Closes issue #33.

```
self = <test_authentication.TestAuthenticateMessage testMethod=test_authenticate_dmarc_prev_spf>

    def test_authenticate_dmarc_prev_spf(self):
        prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com"
>       res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

authheaders/test/test_authentication.py:155:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
authheaders/__init__.py:434: in authenticate_message
    return str(auth_res)
           ^^^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:476: in __str__
    return ''.join((self.HEADER_FIELD_NAME, ': ', self.header_value()))
                                                  ^^^^^^^^^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:492: in header_value
    strs.append(str(result))
                ^^^^^^^^^^^
/usr/lib/python3/dist-packages/authres/core.py:222: in __str__
    strs.append(str(property_))
                ^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <authres.core.AuthenticationResultProperty object at 0x7fc4178465f0>

    def __str__(self):
        if self.comment:
            return '%s.%s=%s (%s)' % (self.type, self.name, self.value.quote_if_needed(), self.comment)
        else:
>           return '%s.%s=%s' % (self.type, self.name, self.value.quote_if_needed())
                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^
E           AttributeError: 'NoneType' object has no attribute 'quote_if_needed'

/usr/lib/python3/dist-packages/authres/core.py:132: AttributeError
```

Author: fumiyas

authheaders/__init__.py

@@ -232,11 +232,13 @@ def dmarc_per_from(from_domain, spf_result=None, dkim_result=None, dnsfunc=None,
         if not policy_only and spf_result and spf_result.result == "pass":
             # The domain in SPF results often includes the local part, even though
             # generally it SHOULD NOT (RFC 7601, Section 2.7.2, last paragraph).
-            try:
-                mail_from_domain = get_domain_part(spf_result.smtp_mailfrom)
-            except IndexError:
-                mail_from_domain = None
-            spf_result.smtp_mailfrom = mail_from_domain
+            mail_from_domain = spf_result.smtp_mailfrom
+            if mail_from_domain is not None and '@' in mail_from_domain:
+                try:
+                    mail_from_domain = get_domain_part(mail_from_domain)
+                    spf_result.smtp_mailfrom = mail_from_domain
+                except IndexError:
+                    mail_from_domain = None
             if aspf == "s" and from_domain == mail_from_domain:
                 result = "pass"
             elif aspf == "r" and get_org_domain(from_domain) == get_org_domain(mail_from_domain):

authheaders/test/test_authentication.py

@@ -145,10 +145,30 @@ def test_authenticate_dmarc_comma(self):
         res = authenticate_message(self.message8, "example.com", spf=False, dnsfunc=self.dnsfunc)
         self.assertEqual(res, "Authentication-Results: example.com; dkim=pass header.d=example.com header.i=@example.com; dmarc=pass (Used From Domain Record) header.from=example.com policy.dmarc=reject")
 
-    def test_prev(self):
+    def test_authenticate_dkim_prev_spf(self):
         prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com"
-        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dmarc=False, dnsfunc=self.dnsfunc)
-        self.assertEqual(res, "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass header.d=example.com header.i=@example.com")
+        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=False, dnsfunc=self.dnsfunc)
+        self.assertEqual(res, f"{prev}; dkim=pass header.d=example.com header.i=@example.com")
+
+    def test_authenticate_dmarc_prev_spf(self):
+        prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com"
+        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
+        self.assertEqual(res, f"{prev}; dkim=pass header.d=example.com header.i=@example.com; dmarc=pass (Used From Domain Record) header.from=example.com policy.dmarc=reject")
+
+    def test_authenticate_dmarc_prev_spf_mailfrom_at_domain(self):
+        prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=bogus@gmail.com"
+        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
+        self.assertEqual(res, f"{prev.replace('bogus@', '')}; dkim=pass header.d=example.com header.i=@example.com; dmarc=pass (Used From Domain Record) header.from=example.com policy.dmarc=reject")
+
+    def test_authenticate_dmarc_prev_spf_mailfrom_at_invalid(self):
+        prev = "Authentication-Results: example.com; spf=pass smtp.mailfrom=bogus@"
+        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
+        self.assertEqual(res, f"{prev}; dkim=pass header.d=example.com header.i=@example.com; dmarc=pass (Used From Domain Record) header.from=example.com policy.dmarc=reject")
+
+    def test_authenticate_dmarc_prev_spf_no_mailfrom(self):
+        prev = "Authentication-Results: example.com; spf=pass"
+        res = authenticate_message(self.message2, "example.com", prev=prev, spf=False, dkim=True, dmarc=True, dnsfunc=self.dnsfunc)
+        self.assertEqual(res, f"{prev}; dkim=pass header.d=example.com header.i=@example.com; dmarc=pass (Used From Domain Record) header.from=example.com policy.dmarc=reject")
 
     def test_get_domain_part(self):
         froms_to_test = [['test@example.com', 'example.com'], [""""Test, User" <test@example.com>""", 'example.com'], ["""Test User <test@sub2.example.biz>""", 'sub2.example.biz'], ["""=?UTF-8?B?QmVkIEJhdGggJiBCZXlvbmQ=?=<BdBth&Byond@example.com>""", 'example.com'], ]