Re: [bitfolk] 21 critical Exim security issues need addressing

Prior to emailing the list, I tried several combinations of hosts and paths before giving up and just using deb.debian.org. Even after your explanation above, I'm unsure how to derive the correct cached source URL, given that the official backport instructions don't give a source URL which uses ftp.uk.debian.org or backports.debian.org (or any of the other mirrors listed in the wiki page). Maybe there's an underlying assumption that a mirror can be used simply by swapping out the original host name for the mirror, but even if that's the case I think it should be documented explicitly because it's not uncommon for mirrors to be hosted at a different URL path to where they were on the original host.