Email sent to team members with Comcast.net addresses blocked by "policy"

Any suggestions on how to get Comcast to allow our hockey league domain to communicate with players? Every message we send to a Comcast.net account is rejected. 

Message blocked
Your message to [deleted]@comcast.net has been blocked. See technical details below for more information.
The response from the remote server was: 552 5.2.0 JxpuxOqT2EZyCJxpvxPpHc Prohibited by policy

Gmail generates the error message, fwiw. 
Thanks.