A pretty decent overview of the scope of the product.

As mentioned in the video, the service also is involved if your app does HTTP over port 80, offering at least some marginal level of improvement. Otherwise it leaves your app traffic as is.

As to Mail, the linked comment mentions that but I don't remember it being a part of the solution (nor does it seem feasible that it could be). Apple offers privacy improvements in mail, but not via the private relay.