Call to undefined function variable_get()
It seems you are confusing Drush 8 and 9 - Drush 8 has vget and vset. 

Using Drush 8, the following two commands worked:

drush vget "mail_system"

drush ev 'print_r(variable_get("mail_system"));'


PS! Drush 7 isn't supported and you should use Drush 8 (currently 8.1.17) for Drupal 7.