On Fri, 6 Sept 2024 at 04:16, Damien Miller <djm@mindrot.org> wrote:
> The only problem is that someone, somewhere, somehow will have used '@'
> in a hostname and this will break them.

Now we both know that this will not prevent anyone out there from
using it, but isn't an '@' in a hostname formally invalid?
RFC 952 and 1123 specify \-0-9A-Za-z and essentially A-Z is treated as
a-z and the period separates components. The hostname also may not
start with a hyphen.

Even if an '@' for some reason would one day be a valid within a
domain name, it probably would have to be punycode encoded.
DNS names on the other hand may contain any octet (RFC 2181, section
11), but that would then probably prevent them from serving as
hostnames.