Stop emails going into spam box

When sending emails via script, the emails are going into spam box of users, we would like someone with experience that can stop emails from going into spam box. Additionally on sign up page, we register the email address and provide login links to hotmail, yahoo, gmail, when email address is recorded to display the login details, (you can see these links by registering at [login to view URL] Coder also to write comments in their code.