As a music supervisor I get 100-200 emails a day so there are only three reasons I will listen to an unsolicited track.
1) Somone I know and trust has sent me a track or given my name and contact info with an opening line of (someone I know) told me to send you this
The music business is a people business so we all have friends and friends of friends that we give a most favored listen to. But in the end no professional will put in bad music that doesn't work for a project. Friendship give you a listen and nothing more.
2) You have an amazing photo or tag line.
A photo of the band that captures my imagination either because it's so strange or compelling that I need to hear what this band sounds like. Also by tag line I mean an artist version of an elevator pitch like "Her voice sounds like a buzzsaw dipped in honey and lit on fire" What the heck does that sound like? I don't know but I am going to click the link to find out.
3) You have done what most artists are too busy or too lazy to do... your homework.
You have actually done some research into the projects they are working on. While I am focused on helping other music supervisors I still do a few small indie projects every year. But if you are sending stuff to a supervisor for a series watch the show and see what kind of music they are using. Make sense? You can also put in your email I loved the scene where you used... it really made the show. I think my song ... would work in the same way. DO NOT EMAIL AND SAY WHAT ARE YOU LOOKING FOR!!!! They hate that.
Good luck and remember these people love music as much or more than you do.