If you're emailing potential VO buyers, make sure the subject header is a question and close your email with a question. For example, in the subject header, use something like..."Are you looking for voice actors?" And in the email, you should close with something like, "Would you mind if I send an mp3 of my demo?" The client now feels compelled to respond. But, if you close out the email with, "Thank you for your time," you've let them off the hook and more times than not, they are not going to reply.
This type of email marketing has worked very well for me and is most effective with production companies. However, with talent agencies, it's essential to follow their protocol as to how they want your demo submitted. I'll be blogging more about talent agency marketing soon!