Thanks for providing a clear context!
Regarding the account itself, unfortunately without the recovery kit it won’t be possible to access the account… the passphrase itself won’t be enough even if the smtp settings were correct because the private key is never stored on the server.
It is still possible to retrieve shared passwords from the other individuals or groups but in this particular scenario you’d need to create a new account, at least for the reconfiguration of the smtp settings.
sudo su -s /bin/bash -c "/usr/share/php/passbolt/bin/cake passbolt register_user -i" www-data
Prior to the creation, please don’t forget to use the role admin for this account since we will need to reconfigure the smtp settings through the administration.