Comcast only allows 100 recipients per email, which includes the To: CC: and
BCC: fields.
"The Comcast SMTP servers have a number of automated anti-Spam measures in
place. In addition to the 1000 message limit [per day]..., there are limits
on how many recipients per email (100) and how many emails per connection
(20). There are also limits on how many times you can hit send within 1
minute."