It could be useful but the entity behind this service would need to be reputable with a proven track record. Because what this amounts to--from an end user pers

It could be useful but the entity behind this service would need to be reputable with a proven track record. Because what this amounts to--from an end user perspective--is a breach of privacy. As for the technical part, I think you're better off passing a callback URL to the service so it posts to it when the user verifies the email. The callback URL could be an AWS lambda.