How can I use a plugin DLL in Editor (for tooling) but exclude it from my build?

I have an external mono dll that I need to use in Editor scripts as part of my tooling path but I don’t want it included in my iOS build. Placing the dll in Assets/Plugins works for Editor but it gets picked up for the build (and fails to cross compile). It isn’t required by the runtime so I’d like to exclude it.

Any pointers on how to go about it?

Thanks

in the Project window, place it like so:

  Plugins
  |_YourNameHere
     |_Editor
        |_YourDLL