I have a Comcast email account. When I send a fresh Comcast email to a particular address account that uses a Google mail server the email goes through and works properly. When I REPLY to an email from the address that uses the Google server, the email doesn’t arrive and Google tech says that the server didn’t even see it. So far the elves from Comcast and Google have been no help. The same thing happens whether replying via Apple mail or direct from Comcast mail website. Please, no finger pointing!!!
Yes. The Reply email shows up in my sent folder. I did try another test. I have a separate gmail account so I sent an email from my gmail account to my Comcast account. I replied to the message from my Comcast account and DID receive it in my gmail account. This makes me believe that the problem is with the specific gmail server for the business account where the reply email is not being received. That being said, Google claims that their server for that business account never sees the reply email.
I think you're right in that it looks like the issue is with Google. Do you have some other non-Comcast and non-Google address like Outlook.com or Yahoo? If so can you get them to send you another message at that account? Then use Reply from there and see if the same thing happens or not.