Hi mdgross,
I am very new to blogs and encountered the exact same problem with Comcast.
This is what worked for me.
I contacted the Comcast Customer Security Assurance Department (formerly Network Abuse) and after they ran a test trying to register on our blog, informed me that “No reverse DNS entry was found”.
I contacted my hosting site who then created a Reverse DNS and within an hour, the registration and lost password emails were no longer blocked. Everything now is working fine with Comcast.
Again, since I am new to this I don’t know if it will address your issue, but I was thrilled to have it fixed.
