Why does yum return error: "[Errno 14] curl#5 - 'Could not resolve proxy: <proxy_servername>; Unknown error' Trying other mirror"?

Issue

• The yum install/update or theyum repolist command fails with error : Could not resolve proxy: <redacted>; Unknown error

# yum repolist
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
https://cdn.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os/repodata/repomd.xml: [Errno 14] curl#5 - "Could not resolve proxy: <redacted>; Unknown error"
Trying other mirror.
https://cdn.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os/repodata/repomd.xml: [Errno 14] curl#5 - "Could not resolve proxy: <redacted>; Unknown error"
Trying other mirror.
repo id                                                                                            repo name                                                                                                 status
rhel-7-server-rpms/7Server/x86_64                                                                  Red Hat Enterprise Linux 7 Server (RPMs)                                                                  0
repolist: 0