[CentOS] Bind 9.3.4-10.P1.el5_3.3 check-names and underscores

Updating old CentOS 4 DNS servers to CentOS 5, one of our zones has a number
of entries with underscores in.  Starting named results in the following
error and named refusing to start.

masters/example.com.db:33: mail_server.example.com: bad owner name
(check-names)

I've tried including

check-names master ignore;

in the options but this isn't making any difference.  I've read a lot of
debate on whether underscores should be allowed, but for the time being
we're stuck with these entries so I need to get this working.

Any thoughts?

Thanks, jc
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