Gracefully handle MX records that do not resolve to an A record

.travis.yml

@@ -12,12 +12,6 @@ env:
 # Matrix approach here due to: https://github.com/travis-ci/travis-ci/issues/9815
 matrix:
   include:
-    - python: 3.4
-      dist: xenial
-      sudo: true
-    - python: 3.5
-      dist: xenial
-      sudo: true
     - python: 3.6
       dist: xenial
       sudo: true

setup.py

@@ -58,8 +58,6 @@ def readme():
         # Specify the Python versions you support here. In particular, ensure
         # that you indicate whether you support Python 2, Python 3 or both.
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.4',
-        'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
     ],

trustymail/__init__.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals, absolute_import, print_function
 
-__version__ = '0.7.4'
+__version__ = '0.7.5'
 
 PublicSuffixListFilename = 'public_suffix_list.dat'
 PublicSuffixListReadOnly = False

trustymail/trustymail.py

@@ -173,9 +173,28 @@ def starttls_scan(domain, smtp_timeout, smtp_localhost, smtp_ports, smtp_cache):
                 # To get around this I look up the A record and use
                 # that instead of the hostname in DNS when I call
                 # smtp_connection.connect().
-                addr_info = socket.getaddrinfo(
-                    mail_server, port, socket.AF_INET, socket.SOCK_STREAM
-                )
+                try:
+                    addr_info = socket.getaddrinfo(
+                        mail_server, port, socket.AF_INET, socket.SOCK_STREAM
+                    )
+                except socket.gaierror:
+                    # We get this exception if there is no A record
+                    # for the given mail server.  This does happen,
+                    # since among their MX records some domains do
+                    # list some IPv6-only mail servers.
+                    #
+                    # Since we can't evaluate such cases we will
+                    # simply log this and give them credit.  One of
+                    # the other mail servers will support IPv4.
+                    error_str = f'The mail server {mail_server} does not have an IPv4 address.'
+                    handle_error('[STARTTLS]', domain, error_str)
+                    logging.warn(error_str)
+                    domain.starttls_results[server_and_port]['is_listening'] = True
+                    domain.starttls_results[server_and_port]['supports_smtp'] = True
+                    domain.starttls_results[server_and_port]['starttls'] = True
+                    continue
+
+                # Extract the IP address from the socket addrinfo
                 socket_address = addr_info[0][4]
                 mail_server_ip_address = socket_address[0]