Steve Atkins wrote:
No, *host names* are scarcely restricted at all. You may wish it were
otherwise, but it's not the case. In particular, underscores are
downright common in hostnames, and most DNS servers don't put any
constraints on them. There are RFC requirements on them, sure, but
most people naming hosts aren't aware of them, they're not enforced by
software and nothing bad happens when you violate them.
About the only thing you can rely on is that most (all?) registries
enforce those character restrictions in domains registered with them.
Which, fortunately, is enough to avoid the _domainkey.com "problem".
People do all sorts of dumb things. Shall we document them all?
NOTE WELL: This list operates according to