Start with the highlights.
If you're emailing someone who isn't familiar with your music, start with a few highlights. Include a SoundCloud link to your best song, or the song you think is a good starting point to make it easy for a blogger who's scrolling through 100 submissions.
We know, we know—ALL your songs are good and we should listen to the entire project as a whole. Stop it. Stop, stop, stop it. It's 2014, and nobody has the attention span for that. Nobody is going to blindly download your entire mixtape, and if they do decide to randomly press play on a song and it happens to be the ONE shitty song on your album, there goes that chance.
Give yourself a shot and pick one song. Or ask a friend whose taste you trust to pick the one song that you send out. Call it a "single." If it's awesome, everyone will want to hear more.