Services like Gmail, Outlook.com and Yahoo! Mail block email to protect you from unwanted or malicious email including phishing, scams and spam. In this case, they are mistaking email that you want for email that you don’t want.
The first step is to ask your email provider to unblock or whitelist an email address. Once that's done, email from that address will be more likely to get through.
Here is an explanation of how to do that for Gmail:
How to whitelist an email address with Gmail.com
Sign in to your account at Gmail.com and do the following.
Add the address to your contacts
Add the email address that is sending you email (eg firstname.lastname@example.org) to your Gmail Contacts. Google will usually deliver email from addresses that are in your Contacts, even if it once recognized them as spam.
Mark messages as ‘Not spam’
If Gmail has marked emails you wish to whitelist as spam, tell Gmail the emails are not spam. Here's how:
- In Gmail, navigate to the spam folder.
- Search for emails containing the domain you wish to whitelist (eg onlinegroups.net).
- Select the emails shown.
- Click More and then select Not spam.
At this point, you should be good go. Please contact support if you have any further questions.