Spinnin is a big label. I'd recommend starting smaller. Look in to some indies before going to the big boys.
I know someone at a major who has a blacklist. He literally writes a list of artists/producers who cross him in a negative way, whether they send in a poor submission, have an attitude, whatever it maybe. He literally has a system which marks any of those names as spam in his emails so he doesn't need to read them.
Start small, if you get in to talks with them, then maybe you have something worthwhile for the likes of Spinnin Records. You can get in contact with them then...
In the process, you've developed experience and some self confidence which will come be very useful to you.
In regards to sending a demo/original mix, whatever you think is quality. Make sure it's
quality. I imagine you don't have any numbers, and you're probably starting out with little radio play. You can gain even more experience by learning the radio submission environment.
Labels nowadays don't invest in artist development, they want the finished product. Try the indies like I mentioned, or build your career your self. When you start making a name for yourself, Spinnin will probably love that you've submitted to them.
Hope I've helped - sorry if I've taken you down a different path. Just a couple of things I think you should consider.
Cheers,
Jordan
P.s. going entirely independent will require a lot of work, I write about forming your own music career on my blog. Feel free to check out the first two posts. You may find them useful.
Why Your Attitude Stinks
Are Your Objectives Smart?