Such email clients as Yahoo, Hotmail, Aol have heavy spam control. And the main reason why your customers do not receive your emails is that email clients your site members are using mark your emails as spam. To resolve this issue you should contact these mail services and ask them to check why your letters are detected as spam, and how to prevent this situation in future.

And if it's Yahoo, Hotmail or Aol - make sure that your members add your general site email to their address books as mentioned on the verification page.

Also, the letter has been detected as SPAM because mail was sent out automatically. So, the letters can get to the SPAM folder.