Emails I send to myself and others with comcast addresses going to Junk

Emails I send to myself from any of my devices are going to my Junk folder in MacMail and also the Spam folder on the xfinity website. This started several months ago and happens when:

1) sending from iPhone 16 Pro

2) sending from my older iMac (Catalina 10.15.8)

3) sending from my 2025 MacBook Air (Sequoia)

This happens with simple text emails and when sending attachments - makes no difference. If it matters, the emails appear in the brown font color in the Junk folder, while others in that window appear in a mix of brown and black font color.

When I log in to xfinity and compose/send within the browser interface, the emails go to the xfinity inbox correctly. However, they continue to go to the Junk folder on my personal devices.

I've spent hours with both xfinity support and Apple support, to no avail, adjusting and resetting Junk mail settings, whitelisting my email address on both the xfinity site and in MacMail, etc. etc. in all permutations.  The "Not Junk" button does not work to train MacMail. I've been hitting it for months. If the email is being marked as Junk by the server, then the Junk mail training in MacMail may not work.

I've also created a Test user account on my laptop, with untouched (default) MacMail settings. I've deleted all previous Rules and Blocks in MacMail. I have also whitelisted my email in my xfinity settings.

In addition, and most importantly, my emails are going to the Junk folders of colleagues who have comcast.net addresses. I feel like there is a server-side filter or setting that is causing this. I have done a soft reset of my router.

Please, can anyone help me? This is creating serious problems for me with my work. Thank you.