>>> From: ludo@gnu.org (Ludovic Courtès)
>
> On the other hand, I just realized that this is a non-standard Emacs
> message mode parens style address described at
> https://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Headers.html
> . guile-email aims to support these non-standard addresses. So, I think
> we can relax the requirement that email headers be strictly ASCII. I'll
> push a fix and let you know.

I've pushed a fix along with a test to prevent regression. See
https://git.systemreboot.net/guile-email/commit/?id=203c9c2ae39a4c64bfd9199bbc2deae03c347998

Could you use a git checkout of the master branch for your guile-email
deployment? Or would you rather I get a minor bugfix release out soon?

Thanks for reporting this bug and thanks for choosing guile-email! :-)