variables in matches?

Can't the built-in variables appear in the match string?
I want
:0
* ^Message-Id:[      ](_dot_)*(_at_)$HOST

instead of
:0
* ^Message-Id:[      ](_dot_)*(_at_)hostname

(NFS mounted home directories & many mail hosts ... sigh)

But $HOST is app'tly interpreted as a string in the first version,
so it never matches.

What can I do?