# enable error logging (either change `/system/config.php` file by hand or use installation wizard step to do it)
# go to Admin Console
# go to {nav Configuration > Website > Regional} section
# open English language for editing
# uncomment commented-out tag at the bottom of "HTML Version" field in "E-mail Design Templates" sub-section
# save changes
# go to {nav User Management > Users} section
# create an active user and remember it's ID
# in the `/index.php` file place this code after `$application->Init();` line: `$application->emailUser('USER.SUBSCRIBE', 3);` (replace `3` with ID of created user)
# open Front-End in another tab
# switch to Admin Console
# go to {nav User Management > Mailings} section
# go to `Email Queue` tab
# confirm, that there 1 record in there
# press `Process Queue` button
# check inbox e-mail address specified during user creation before
# confirm, that it contains the link in the footer, that has non-empty "authkey" url parameter
# go to {nav Logs & Reports > System Log} section
# confirm, that notice about missing `Subject` key isn't present in there