I'm having the same problem, I read about it a little and it might be a comcast server issue that needs fixing. Hard to get in touch with them, if I send a simple 'test' email it seems to be working but emails with links etc... aren't getting through. Your are showing as sent, correct?
I've sent both a PM to gather more info.
The main examples I have are my wife's work email and my daughter's school email which are both not Comcast. The emails show as sent but they never receive them on their end. I assume they are being blocked somewhere. I have to use my work email if I email the as I know they will never receive it with our home comcast email.
I am having a similar issue. emails indicate they are sent (no errors). However they are never received. I did notice that there are no email headers generated when I view the source. Also, when I connect to my VPN for work, the emails will send.