Comcast is the biggest pain in the butt. Its very easy for a domain to get on their blacklist. On my websites, I was constantly getting complaints that users were not getting their emails from us. Turns out that our "Thread Subscribers" had so many emails going to them, that Comcast blacklisted us as spam. After an amazingly silly automated process, I got us un-listed. But I recommend all users to not use Comcast as their registration email provider. Nothing but problems.