Inviting webmasters to a directory

Hi everyone,

We have recently added a directory feature to our website to list other sites in the niche. I was wondering if it is appropriate to send e-mails to webmasters of the websites belonging to that niche with invitation to list their website in our directory?

What do you think? If you got such a letter, would you consider it as a spam?

What are other techniques for promoting a directory?