MX validation behaves inconsistently depending on network connectivity

[afair]

Description: Validation gives inconsistent results depending on network status.

Reproduction:

1. Turn off your network connection.

2. Do the following:

   irb(main):001:0> require 'email_address'
   => true
   irb(main):002:0> EmailAddress::Config.configure(host_validation: :mx)
   => {:dns_lookup=>:mx, :sha1_secret=>"", :munge_string=>"*****", :local_downcase=>true, :local_fix=>false, :local_encoding=>:ascii, :local_parse=>nil, :local_format=>:conventional, :local_size=>1..64, :tag_separator=>"+", :mailbox_size=>1..64, :mailbox_canonical=>nil, :mailbox_validator=>nil, :host_encoding=>:punycode, :host_validation=>:mx, :host_size=>1..253, :host_allow_ip=>false, :host_remove_spaces=>false, :host_local=>false, :address_validation=>:parts, :address_size=>3..254, :address_fqdn_domain=>nil}
   irb(main):003:0> EmailAddress.error('somename@gmail.com')
   => "Domain name not registered"
   

3. Turn your network connection back on.

4. In the same IRB session, with connectivity restored:

   irb(main):004:0> EmailAddress.error('somename@gmail.com')
   => "This domain is not configured to accept email"
   

5. Start a new IRB session:

   irb(main):001:0> require 'email_address'
   => true
   irb(main):002:0> EmailAddress::Config.configure(host_validation: :mx)
   => {:dns_lookup=>:mx, :sha1_secret=>"", :munge_string=>"*****", :local_downcase=>true, :local_fix=>false, :local_encoding=>:ascii, :local_parse=>nil, :local_format=>:conventional, :local_size=>1..64, :tag_separator=>"+", :mailbox_size=>1..64, :mailbox_canonical=>nil, :mailbox_validator=>nil, :host_encoding=>:punycode, :host_validation=>:mx, :host_size=>1..253, :host_allow_ip=>false, :host_remove_spaces=>false, :host_local=>false, :address_validation=>:parts, :address_size=>3..254, :address_fqdn_domain=>nil}
   irb(main):003:0> EmailAddress.error('somename@gmail.com')
   => nil
   

Expected Behavior: Seems like if there's no network connection, it should raise a warning about MX not being able to validate, not report that validation has failed. Also, it definitely shouldn't report "This domain is not configured to accept email" about gmail.com at any point. Whatever is giving that report on that domain is pretty much by definition incorrect.

[afair]

It seems the gem cached the result of the offline response. Not found requests are the slowest and need to be cached. We should see if the result is offline vs not found.