IMail.Save(string path) saves email in EML (MIME) format to disk.
Take a look at byte[] IMail.Render() which returns byte array or IMail.Render(Stream stream) which renders message to specified stream.
Please note that by default IMail.Render() renders empty BCC header. You can use the overload that takes bccRenderMode parameter to change that.
byte[] IMail.Render() // BCC header is *not* rendered.
byte[] IMail.Render(AddressHeaderRenderMode bccRenderMode)
IMail.Render(Stream) // BCC header is rendered.
IMail.Render(Stream, AddressHeaderRenderMode bccRenderMode)