After I posted to the forum, I received a rather detailed response from Karl on this issue. I appreciate the response, thanks. Here it is:
We have been having worsening problems with Comcast blocking our IPs in
the last couple weeks. This is being exacerbated by Comcast's lack of
response to our numerous attempts to contact them to resolve the problem.
When an IP is blocked, Comcast bounces mail with instructions to contact
a specific email address. This email address is automated and removes
blocks on IP addresses contained within a message. The problem is that
these IPs frequently end up blacklisted again, and too many removals will
eventually cause this mechanism to start denying blacklist removal
requests altogether, with absolutely no recourse.
In contacting Comcast's general support (which had no knowledge about the
problem) we retrieved a phone number for their "abuse and security"
division. We've made numerous calls to this number which feeds
invariably into a voicemail.
We have also a couple dozen emails to Comcast's de-listing address as
well as their abuse address requesting some sort of human response on
this issue with information about why mail is being rejected and some
sort of evidence on what kind of mail is generating complaints, so that
we may take appropriate action.
Thus far the response to our requests has been complete silence, and the
occasional automated reply that does not provide any real information
about the problem.
By the looks of it, we're also not alone!
In the meantime I strongly recommend that if you have forwarding
addresses set up to send mail to your Comcast address, you remove these
and set up locally-hosted DreamHost mail users which you can download
mail from via a program like Outlook or Thunderbird, or even check via
webmail. If you need any assistance with this, please contact support
and we will be happy to assist you.
We've also experienced Comcast blocking our listserver IP for
announcement/discussion lists that are set up in our panel. As per
above, we've tried to contact Comcast and explain that this server only
sends to addreses which have opted-in and provides opt-out removal
procedures, but have received nothing but autoresponses. Without
receiving actual spam complaints from Comcast, or without the recipients
taking action to remove themselves from the lists they originally
subscribed to, there isn't a whole lot that can be done to fix this
problem. If you have Comcast recipients on your mailing lists, they may
wish to set up a mail account with a provider that does not blacklist
mail quite so liberally -- just because they use them as an ISP doesn't
mean they have to use them for mail, after all!
Thank you for your continued patience; we are continuing to try and get a
response from Comcast on this issue and hope to eventually rectify it
fully, but above are some methods that you can use in the meantime to
minimize the impact of these blocks on your day-to-day usage.