Email to Groups not working

I use one of my Comcast Secondary Accounts to send group emails. My Army Veterans group has approx. 400 users so I have my group broken into 4 groups to stay under the 100 recipients per email limit.  So when I want to send an email to the group, I actually send 4 emails. This has worked fine for several years until July 14, 2023. Now when I send the emails I see all 4 in my Sent Mail but only a few actually receive the email. I include myself in these groups so that I can see that the email actually went out.

I have a service ticket CR 102371109 but Comcast has not been a help. I wait on hold for a very long time, have to repeat my story over and over and every time my call gets mysteriously disconnected and they never call me back.

I send my emails from my desktop PC either through Outlook to Xfinity.com. I need help and don't where to turn. Can anyone help.

Thanks,

Bill T