There is an ongoing outage on the primary CPAN mirror. It is possible to work around the issue by using MetaCPAN as a mirror.
To install RT::Extension::CommandByMail, copy and paste the appropriate command in to your terminal.
cpanm
cpanm RT::Extension::CommandByMail
CPAN shell
perl -MCPAN -e shell install RT::Extension::CommandByMail
For more information on module installation, please visit the detailed CPAN module installation guide.