emailing for business directory leads

i didnt see any subforum for email so i posted this here, please move if there is somewhere more appropriate for this thread

i have just finished development on a business directory/review site for a niche, I would lie to ask businesses in this niche to fill out the registration and get a listing on this site.

whats the best way to do this? I am going to send out a lot of emails, i am not sure if i need to send them out individually or through a service ( i am just looking at mailchimp, seems good and free) i dont want to be seen as spam,

If any one has experience of this or can show me a useful thread that would be awesome of you