Occasionally a valid email address will get accidentally blocked if marked as spam by the user. Here are the steps to remove the block:
- Open a web browser and go to mail."yourdomain.com" and log in using your email address and email password
- Click on Settings (gear icon) and then expand "My Settings" and "Filtering"
- Click on "Content Filtering" and edit the "Internal Blocked Senders" list
- Go to the next screen and search the "From Address" box by clicking in the box and searching using crtl-F and the email address in question
- Delete the address, then click Next and Save