Email messages not being received by Comcast customers

We are a Club and we send out notices to our owners, many of which are comcast customers.  Sometime last year, those messages stopped being received.  I have tested and captured packets and have determined that the messages are still being sent and accepted by comcast's email servers but then nothing happens.  They just disappear.

Could you please put me in contact with the department that could help me determine why these messsages are not being delivered?  

I have tried many different avenues over the past few months trying to get this resolved and they have all been dead ends.

The emails are being sent by the SMTP Server at smtp.sendgrid.net

Thank you,

Helen