I just realized this is an X-Y problem. The flag is on-the-wire as a part of the M macro.
> is it possible, in principle, to define new milter macros that are > passed to smtpd_milters? [1] does not list a macro that carries > information whether SMTPUTF8 is set. [2] mentions SMFIC_MACRO, but I > have no idea if that is what I think it might be.
