Fix the returned implicit MX record when there is a fallback

The object returned by validate_email returns the queried MX records when deliverability checks are run. When there is an implicit MX record (no MX record but an A or AAAA record), the value is a single entry that points to the host, not a list of the A or AAAA values. SMTP 5321 5.1:

> If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX R, with a preference of 0, pointing to that host.

CHANGELOG.md

@@ -1,6 +1,7 @@
 In Development
 --------------
 
+* When a domain name has no MX record but does have an A or AAAA record, the mx field in the object returned by validate_email incorrectly held the IP addresses rather than the domain itself.
 * Fixes in tests.
 
 2.1.1 (February 26, 2024)

email_validator/deliverability.py

@@ -60,7 +60,7 @@ def validate_email_deliverability(domain: str, domain_i18n: str, timeout: Option
             # If there was no MX record, fall back to an A record. (RFC 5321 Section 5)
             try:
                 response = dns_resolver.resolve(domain, "A")
-                deliverability_info["mx"] = [(0, str(r)) for r in response]
+                deliverability_info["mx"] = [(0, domain)]
                 deliverability_info["mx_fallback_type"] = "A"
 
             except dns.resolver.NoAnswer:
@@ -69,7 +69,7 @@ def validate_email_deliverability(domain: str, domain_i18n: str, timeout: Option
                 # (It's unclear if SMTP servers actually do this.)
                 try:
                     response = dns_resolver.resolve(domain, "AAAA")
-                    deliverability_info["mx"] = [(0, str(r)) for r in response]
+                    deliverability_info["mx"] = [(0, domain)]
                     deliverability_info["mx_fallback_type"] = "AAAA"
 
                 except dns.resolver.NoAnswer as e:

tests/test_deliverability.py

@@ -14,7 +14,7 @@
     'domain,expected_response',
     [
         ('gmail.com', {'mx': [(5, 'gmail-smtp-in.l.google.com'), (10, 'alt1.gmail-smtp-in.l.google.com'), (20, 'alt2.gmail-smtp-in.l.google.com'), (30, 'alt3.gmail-smtp-in.l.google.com'), (40, 'alt4.gmail-smtp-in.l.google.com')], 'mx_fallback_type': None}),
-        ('pages.github.com', {'mx': [(0, '185.199.108.153'), (0, '185.199.109.153'), (0, '185.199.111.153'), (0, '185.199.110.153')], 'mx_fallback_type': 'A'}),
+        ('pages.github.com', {'mx': [(0, 'pages.github.com')], 'mx_fallback_type': 'A'}),
     ],
 )
 def test_deliverability_found(domain, expected_response):