I'm replying to clients that I've dealt with frequently and all of a sudden they're not receiving reply e-mails, they've checked spam folders so that isn't the issue. Through SPAMHAUS, Comcast has blacklisted the BTL IP address any other providers using SPAMHAUS will also be blacklisting e-mails coming through BTL.

This only applies if you are using an e-mail program (Outlook, Eudora, etc.). If you send through an online account there is no issue.