Re: Underscores in host names

[Tony Finch <dot () dotat at>]

There are also mail domains to consider. They have superficially the same
syntax as host names (they cannot have a trailing dot) but they are
generally checked much more strictly for conformance to that syntax. I'm
not sure whether the original post was about a mail domain or the name of
a mail host, but if it was the former I would be surprised if the customer
could claim that it works most of the time.

Names of mail hosts are another matter, especially the names they declare
in HELO. When I analysed this back in December, I found that about 1/3 of
legitimate mail hosts declared invalid hostnames. This is orthogonal to
the issue of host name syntax, but it does show that being excessively
strict will cause you pain. However it is worth checking mail host names
for gross syntax violations since some fairly common spamware puts all
sorts of binary junk in its HELO command.

Tony.
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