Actually most recruiters are looking for job boards with a niche, instead of big and popular job boards that brings a lot of different profiles of job seekers.
You should pitch your niche as the strongest advantage against traditional job boards, because in this kind of market the motto is quality over quantity.
What I strongly suggest at the beginning is to curate your first vacancies, ask your users to reference your job board in the contacting e-mail with the recruiters, and try to keep data of what vacancies are receiving the most profiles or views in your site.
So you can target those recruiters as your first costumers.
PS: You can create a custom "mailto link" with a reference of your job board. https://stackoverflow.com/questions/4782068/can-i-set-subject-content-of-email-using-mailto