I consulted with one of our email experts here on our support team, and ze had this to say:
First guess, ComCast blocks port 25; We provide an alternate port 587 you can use instead. Port details in this wiki page:
Second guess, some ISPs require all outgoing mail use the ISP's SMTP server for outgoing mail, regardless of the incoming mail server. You'd have to check with the ISP for that.
But because your problem is limited to just one internet connection, and you can send fine when using other internet connections, sounds like the issue is probably not with us.
If you'd like us to take a look though, please submit a ticket here with details of a specific email that didn't send (sender & recipient email addresses and the date&time you tried), and we can see if we can figure something out for you!