We have received two (2) client complaints about email being rejected by Comcast.
At the moment we aren't seeing issues with delivery to Comcast, none of our IP addresses on any outbound networks are on an RBL (mxtoolbox.com), and manual telnet SMTP diagnostics tests are so far showing no issues either. This leads us to believe that it's a typical Comcast issue (rejecting mail randomly from different servers as they get swamped and overloaded during peak hours). Best practices for sending mail to Comcast or Yahoo is to just resend the message and hope a different server in their infrastructure accepts the message. When there are RBL issues, we immediately open a support request and start working with their postmaster staff.