My site sends out e-mails and they go to spam folders! need help by nickmarks

Hi, need help solving a problem on my site. Recently just started up a social networking site using PHPFox as my code base. Users can invite other people to site. But these invites are automatically going into other users junk folders...yahoo, etc... (Budget: $30-250, Jobs: Java, PHP, Web Promotion, Website Design)
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