emails: fix deliverability check when MX lookup returns nonexistent domain.

Fix the email deliverability check to return undeliverable when the MX lookup returns a domain or IP
address that is invalid or that refuses connections. For example, `yeah.com` returns the invalid IP
address 0.0.0.0 for its MX record.

app/logical/danbooru/email_address.rb

@@ -349,6 +349,9 @@ module Danbooru
           response = conn.rcptto(address) rescue $!
           return response.is_a?(Net::SMTPFatalError)
         end
+      rescue Errno::ECONNREFUSED
+        # nobody@yeah.com (MX: 0.0.0.0)
+        true
       rescue
         false
       end