Email services like Gmail, Outlook.com and Yahoo! Mail try to protect you from unwanted or malicious email such as phishing, scams and spam by blocking certain email. Unfortunately, they sometimes mistake email that you want for email that you don’t want.
When you ask your email provider to unblock or "whitelist" an email address, email from that address will be more likely to get through.
Here is an explanation of how to do that for the major providers Gmail, Outlook.com and Yahoo! Mail.