Add HostDomain to handle address with underscore. Underscore are allowed in domain by RFC1033 [1][2][3].
HostDomain
[1] https://www.ietf.org/rfc/rfc1912.txt
[2] https://www.ietf.org/rfc/rfc1033.txt
[3] http://domainkeys.sourceforge.net/underscore.html
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