Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/InRelease...
openpgp:  RSA key ID 7638D0442B90D010
openpgp: Good signature from "Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>"
openpgp:  RSA key ID 04EE7237B7D453EC
openpgp: Good signature from "Debian Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>"

Mirror [mirror9]: http://cdn-fastly.deb.debian.org/debian/ stretch-backports successfully added.
You can run 'aptly mirror update mirror9' to download repository contents.