0.1.6:
- Fixed a rare case where aol.co returns '.' in MX that causes an
  exception for the SMTP module, with added tests.

0.1.5:
- Handle socket errors (https://docs.python.org/3/library/
  socket.html#exceptions)

0.1.4:
- Handle 'No MX record' exception

0.1.3:
- Added ambigious (4xx) response code handling
