docs: def substitution(self, substitution):
          """Add substitutions to the email
  
          :param value: Add substitutions to the email
          :type value: Substitution, list(Substitution)
          """
          if isinstance(substitution, list):
              for s in substitution:
                  self.add_substitution(s)
          else:
              self.add_substitution(substitution)