Email providers (Gmail, Yahoo, AOL, etc.) each have their own algorithms for detecting spam, some are stricter than others, some are more accurate than others, but almost every email provider gives an option to whitelist a domain or email. Whitelisting ensures that all mail sent from a specific domain or email address will be successfully delivered into one's inbox.
While we continually work with our server admins to improve deliverability across the various email providers, there is no foolproof way to ensure deliverability, except whitelisting. Please feel free to share this information together with the link below with anyone who is having trouble receiving messages: