If you're using Gmail, you could try this How to set Gmail Spam Filter:

​Step 1. Log in to your Gmail account.

​Step 2. Click the GEAR icon at the top right and then click SETTINGS.

​Step 3. Go to FILTERS AND BLOCKED ADDRESSES and click CREATE A NEW FILTER.

​Step 4. In the FROM section, type the email address of the sender that you want to keep out of your Spam Folder.

​Step 5. Click CREATE FILTER.

​Step 6. Check NEVER SEND IT TO SPAM and ALSO APPLY FILTER TO MATCHING MESSAGES boxes.

​Step 7. Click CREATE FILTER again.