Gpgrelay is an encrypting / decrypting proxy for any MUA for Windows.
This is a useful tool but it is no longer maintained and does not work with GnuPG > 2.1.
It is free software so we could pick it up and fix it. But it is not written in our style and the current build system depends on MSVC. Still we might want to fix it and put it into gpg4win-tools or so.