---
features:
  - |
    Add ``HostDomain`` to handle address with underscore. Underscore are
    allowed in domain by RFC1033 [1][2][3].

    - [1] https://www.ietf.org/rfc/rfc1912.txt
    - [2] https://www.ietf.org/rfc/rfc1033.txt
    - [3] http://domainkeys.sourceforge.net/underscore.html