Email bouncing back from Comcast

I am trying send email to someone with a comcast account that I have emailed several times before in the past and now they are bouncing back with this message.

BL000000 :: Mail to Comcast is rejected and is returned with an error message containing the code BL000000. What does this mean?

Our filters have determined that email from your email server has been sent in patterns which are characteristic of spam. In an effort to protect subscribers, your email server has been blocked from sending email to the Comcast network. Mail servers are typically shared by many users so it may be the case that another party using your email server has sent spam, even if you have not.

How can this be remedied?

Thank you,