Throttled Email issues to Xfinity Clients

Our work email is stated by Xfinity support to have been flagged due to our Domain and we cannot email a client who uses a Comcast.net email.

All items have been reviewed and our URL has zero issues and just spent a pretty penny to get that reviewed and additional security added.

Error code is "451 4.2.0 Throttled" - we use Google Workspace and per our client, they are pointing the finger at our URL, yet we can't find a single issue on our end domain wise or IP address wise.

The IP address they claim is ours and it is not ours - so how do we get our email off this Xfinity blacklist so we can email our comcast client again? It's someone we have to email daily. The last three days our email is 1/4 of these throttled email responses. This isn't a personal email - it's our work email and we need to get this fixed.