Legit emails to our customers who have comcast.net email addresses never get delivered

First off, I'd like to preface this by saying that I originally tried PMing this to an employee on here, but got the "Authentication Failed.  Authentication Ticket Mismatched, failed authentication" error.  No idea what that's all about, since I was signed in, but now I'm turning to the forum.  Unfortunately, because I'm a small business customer and not an Xfinity customer, I can't just PM ComcastAntiSpam like I'd want because he's been dealing with my same problem and getting things done for the Xfinity people.

I've removed all the personal details, but can provide them if someone PMs me.

For the last 2-3 months, our domain has had problems emailing job estimates to customers that have requested roofing and siding bids from us.  We have no trouble whatsoever getting those same emails to Gmail, Yahoo, .edu emails., etc.  It's comcast.net that's consistently giving us problems.  We don't do mailing lists, we don't do spam.  The only reason any of these people get anything from us is because they've asked us to email them, and once business is concluded, we tend not to contact them ever again, unless they reach out to us.  Sometimes there is an attachment, sometimes there isn't--they are generally 1MB or less and in either a .jpeg, .png,, or .pdf format.  There is no error message that we get, no bounce back, nothing.  It gets sent, says it gets sent, and then never arrives on the other end.

Everything worked fine, to the best of our knowledge, last year.  We've done nothing to change anything, so it's definitely a Comcast problem, especially since the forums are full of complaints about the same thing.  As far as I can tell, we're on a dynamic IP.  We do our email through webmail at server308.com, which is either accessed directly through that service or received through Thunderbird or Outlook 2010.  We have about 6-7 computers running Windows 7.  We don't do our email directly through Comcast because we're a recent switch over and that's not how things were set up.  We shouldn't have to and considering how things are working out right now, I don't think we would ever WANT to.

I've checked out Spamhaus.org and my specific IP is not on the SBL or XBL lists, but it IS on the PBL list, which in this case is PBL1523198.  I don't know if that has anything to do with it or not.  SMTP authentication appears to be set correctly for all places concerned and has been all along.  I emailed the people maybe involved with this sort of thing at Cips_AbuseAdmin@cable.comcast.com on 5/15/19, but haven't heard from anyone.  The content of the email was essentially what is here, minus the technical stuff because at the time, I didn't realize it was going to get this involved.  It's possible the email didn't even get to them, because it's a Comcast email.  I sent an unblock request this morning and got the response below.

The IP that I gave them was what I had--I could get the IPs for the other computers, but I'm not sure it would do any good.  They all have this problem, which to me says it's being blocked somewhere.  As mentioned before, we're not going through the Comcast website for our email, so there is no 550 error code, or any error code or message, to list.

This has been a painful experience with no end in sight.  Who knows how many thousands of dollars we've lost in business because of it.  Please advise.