configuration_is_ignored.rb

  class Invitation < ActiveRecord::Base
    belongs_to :invitable,  primary_key: :email, foreign_key: :invitable_id, class_name: 'User'
  end

  class User < ActiveRecord::Base
    has_many :invitations, :primary_key, :email, foreign_key: :invitable_id
  end

  user = User.create(email: '[email protected]')
  invitation = Invitation.create(invitable: user)
  
  invitation.reload

  invitation.invitable_id == user.email # false
  invitation.invitable_id == user.id # true

Maybe it's the typos in your snippet: :primary_key, :email...? This works for me:

migration:

class UserInvitation < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :email
      t.timestamps
    end

    create_table :invitations do |t|
      t.string :invitable_email
      t.timestamps
    end
  end
end

user.rb:

class User < ActiveRecord::Base
  has_many :invitations, foreign_key: :invitable_email, primary_key: :email

  validates       :email, :presence => true, :uniqueness => true
  attr_accessible :email
end

invitation.rb:

class Invitation < ActiveRecord::Base
  belongs_to :invitable, class_name: 'User', foreign_key: :invitable_email, primary_key: :email

  attr_accessible :invitable
end

in the console:

user = User.create(email: '[email protected]')
invitation = Invitation.create(invitable: user)
invitation.reload

invitation.invitable_email == user.email # true