Get existing certificate by user's email address

Hi,

In our app, we have a flow like this:

User answers the questions.
User types in their email address.
If they don't have the certificate, we issue a new one. If user already has the certificate, we display the existing one.

Looking at the documentation there is no way to find the certificate by email. GET endpoint requires the certificate ID which we do not have at that point. List credentials endpoint does not provide any filtering options in the docs.

I could request all the credentials and filter them on my side, but if we want to have thousands of them, this will be very expensive for your service and my service as well.

Is there an option to get the credential by email address that I dont know of? And if there is no such way, can we add filtering options to 'list credentials' endpoint?