What I strongly suggest at the beginning is to curate ur first vacancies, ask ur users to recommend ur job board n target the most popular vacancies in ur site.
I think the better strategies would be to just crawl/aggregate their jobs automatically and post it on your niche job site instead of asking the recruiters to post it. Just do their job for them until you have a lot of users and the recruiters will start doing it on their own.
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