additional_message_headers List-Unsubscribe

[soonsoo]

Installed roundcube 1.5.1.

Use the additional_message_headers plugin and I want to put the user's email address in List-Unsubscribe .

But $_SESSION['username'] outputs an empty value.

$config['additional_message_headers']['List-Unsubscribe'] = '<mailto:'.$_SESSION['username'].'?subject=Unsubscribe>';

Is there no way? T.T