I have a Comcast cable modem and Outlook Express for email.
I can recieve, but not send.
I just had a similar problem and it turned out that Trend Micro was set to scan outgoing messages yet it wasn’t actually sending them. I turned off the function that makes it scan outgoing.
I’m not a computer whiz so that’s all I got. Sorry. 
Could be one of several things.
Configuration, port setting, secure outgoing connection, without that we can’t do much.
Suggest you call your ISP and ask them for the email settings.
First thing that comes to mind is to check your mail settings. More specifically, your SMTP settings. If you’re using another party’s email (i.e. your email address doesn’t end with “comcast.com,” then you need to specify them in your settings as the outgoing (SMTP) server.
Also, some SMTP servers require authentication before you can send to prevent anonymous connections. I’m not sure about Express, but in regular Outlook, go to
Tools>Email Accounts>View or change Existing email accounts
Select the account, click “change”. Click “More Settings”, go to Outgoing Server tab, and check “My Outgoing Server requires authentication.”
Here is how to configure Outlook Express for your Comcast account: