The newly added (and package masked) mail-filter/libmilter-8.13.7 is redundant. Milters require sendmail to be installed, and mail-mta/sendmail-8.13.7-r2 unconditionally installs libmilter. Therefore a separate libmilter ebuild is not only not required but will cause conflicts if it is installed.
Please, leave the thing alone. It's package.masked meanwhile, and has been committed for a reason.