Changed line 27 from: use Zend\Mail\Protocol;
Changed line 27 from:
use Zend\Mail\Protocol;

to:
use Zend\Mail\Protocol,
    Zend\Mail\Storage;

As the declaration of the static variable $_knownFlags triggered the error:
Fatal error: Class 'Zend\Mail\Storage\Storage' not found because the autoloader tried to find the class Storage inside the name space Zend\Mail\Storage.
basz/zend-mail,matbech/zend-mail,rapidsoft-de/zend-mail,MadCat34/zend-mail,mleko/zend-mail,Xerkus/zend-mail,stefanotorresi/zend-mail