I had a similar experience when sending to a Gmail user via a forwarding service. Gmail spotted that an email sent from a TalkTalk address was coming in from a non-TalkTalk server. I now use the recipient's native Gmail address instead of the forwarding service, and Gmail does not reject. Not saying this is your situation, but it might be.
Now surfing from a Dell Inspiron 3881 running Windows 11 and a Qualcomm QCA9377 Wireless Network Adapter with Sagemcom FAST 5364 router.
