The unrealircd.org/userhost message tag communicates the original user@host
to IRC Operators so they can see the uncloaked host.
Normal users will not see this tag, only IRCOps.

Syntax of the tag:
unrealircd.org/userhost=<user>@<host>

Example line:
@unrealircd.org/userhost=~user@localhost :nick!~user@Mask-ABCDEF PRIVMSG #test :hi

See also other IRCOp-only "informative" message tags:
* unrealircd.org/userip
* unrealircd.org/geoip