As far as sending email, I am not sure, but it sounds like the same issue I had.
I also had a cable internet, I also had set email server to use cable company email server. Everything worked fine for a while, than all over sudden we were unable to send out any emails.
After few calls to the cable company, I was informed that I am not allowed to have my own email server if I have a residential internet account (against TOS), only way to be allowed to have my own server is to order a business cable account.
I have read in some other forums that their block can be bypassed by using different email server port, but I was unable to get it to work. Only way I got it to work was by moving my server to a server warehouse (co-location facility)