Most likely the reason the client cannot receive emails, but can send email, is because their Email Settings have "Email Safe List" enabled "On". 

To verify, please have them login to their account and turn this feature to "Off", or if they want to keep this feature "On", edit the Email Safe List to allow sender email addresses of your choice.   

Steps to check and/or edit Email Safe List: 

  • Login to Xfinity Connect Web from your browser-> 
  • Click on Sign In Sign In Click My Account Click on Users & Preferences 
  • Click Email Settings and modify your Email Safe List to Off, or On (but, you'll need to select edit to explicitly allowed senders).

Here is a comcast article with more detail: