Here is a link with details on blocked ports. Per the article:
Inbound and outbound, not blocked by default. We may apply a sending block, which does not interrupt Comcast webmail service. However, it will prevent email programs or clients (e.g., Outlook Express) from sending email.
An unsecured port that can be used to send spam. Customers may be advised by our Security Assurance team to switch their modem connection to a secured port that requires authentication (such as port 587).
In very rare situations, port 25 will be blocked by Comcast Customer Security Assurance on a per customer basis (blocked at the modem) and not across the network. This block will be preceded by an email and letters to the billing address. In this case, you can use secured port 587 for sending email. Get instructions to configure your email client to send via a secured port.
If you are running a mail server please contact Comcast Customer Security Assurance at 1-877-807-6580 for more information on this block.