diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1408cb5..49b3aad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ scheme are considered to be bugs.
 
 ### Added
 
+* [#499](https://github.com/hashie/hashie/pull/499): Add `Hashie::Extensions::Mash::PermissiveRespondTo` to make specific subclasses of Mash fully respond to messages for use with `SimpleDelegator` - [@michaelherold](https://github.com/michaelherold).
 * Your contribution here.
 
 ### Changed
diff --git a/Gemfile b/Gemfile
index 524b968..5001638 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,6 +4,7 @@ gemspec
 
 group :development do
   gem 'benchmark-ips'
+  gem 'benchmark-memory'
   gem 'guard', '~> 2.6.1'
   gem 'guard-rspec', '~> 4.3.1', require: false
   gem 'guard-yield', '~> 0.1.0', require: false
diff --git a/README.md b/README.md
index 9bbaebc..f52f1f2 100644
--- a/README.md
+++ b/README.md
@@ -658,6 +658,30 @@ mash['string_key']  #=> 'string'
 mash[:string_key]  #=> 'string'
 ```
 
+### Mash Extension: PermissiveRespondTo
+
+By default, Mash only states that it responds to built-in methods, affixed methods (e.g. setters, underbangs, etc.), and keys that it currently contains. That means it won't state that it responds to a getter for an unset key, as in the following example:
+
+```ruby
+mash = Hashie::Mash.new(a: 1)
+mash.respond_to? :b  #=> false
+```
+
+This means that by default Mash is not a perfect match for use with a SimpleDelegator since the delegator will not forward messages for unset keys to the Mash even though it can handle them.
+
+In order to have a SimpleDelegator-compatible Mash, you can use the `PermissiveRespondTo` extension to make Mash respond to anything.
+
+```ruby
+class PermissiveMash < Hashie::Mash
+  include Hashie::Extensions::Mash::PermissiveRespondTo
+end
+
+mash = PermissiveMash.new(a: 1)
+mash.respond_to? :b  #=> true
+```
+
+This comes at the cost of approximately 20% performance for initialization and setters and 19KB of permanent memory growth for each such class that you create.
+
 ### Mash Extension: SafeAssignment
 
 This extension can be mixed into a Mash to guard the attempted overwriting of methods by property setters. When mixed in, the Mash will raise an `ArgumentError` if you attempt to write a property with the same name as an existing method.
diff --git a/benchmarks/permissive_respond_to.rb b/benchmarks/permissive_respond_to.rb
new file mode 100755
index 0000000..e7e894e
--- /dev/null
+++ b/benchmarks/permissive_respond_to.rb
@@ -0,0 +1,44 @@
+#!/usr/bin/env ruby
+
+$LOAD_PATH.unshift File.expand_path(File.join('..', 'lib'), __dir__)
+
+require 'hashie'
+require 'benchmark/ips'
+require 'benchmark/memory'
+
+permissive = Class.new(Hashie::Mash)
+
+Benchmark.memory do |x|
+  x.report('Default') {}
+  x.report('Make permissive') do
+    permissive.include Hashie::Extensions::Mash::PermissiveRespondTo
+  end
+end
+
+class PermissiveMash < Hashie::Mash
+  include Hashie::Extensions::Mash::PermissiveRespondTo
+end
+
+Benchmark.ips do |x|
+  x.report('Mash.new') { Hashie::Mash.new(a: 1) }
+  x.report('Permissive.new') { PermissiveMash.new(a: 1) }
+
+  x.compare!
+end
+
+Benchmark.ips do |x|
+  x.report('Mash#attr=') { Hashie::Mash.new.a = 1 }
+  x.report('Permissive#attr=') { PermissiveMash.new.a = 1 }
+
+  x.compare!
+end
+
+mash = Hashie::Mash.new(a: 1)
+permissive = PermissiveMash.new(a: 1)
+
+Benchmark.ips do |x|
+  x.report('Mash#attr= x2') { mash.a = 1 }
+  x.report('Permissive#attr= x2') { permissive.a = 1 }
+
+  x.compare!
+end
diff --git a/lib/hashie.rb b/lib/hashie.rb
index a3fc11c..7f88ed4 100644
--- a/lib/hashie.rb
+++ b/lib/hashie.rb
@@ -45,6 +45,7 @@ module Hashie
 
     module Mash
       autoload :KeepOriginalKeys, 'hashie/extensions/mash/keep_original_keys'
+      autoload :PermissiveRespondTo, 'hashie/extensions/mash/permissive_respond_to'
       autoload :SafeAssignment, 'hashie/extensions/mash/safe_assignment'
       autoload :SymbolizeKeys, 'hashie/extensions/mash/symbolize_keys'
       autoload :DefineAccessors, 'hashie/extensions/mash/define_accessors'
diff --git a/lib/hashie/extensions/mash/permissive_respond_to.rb b/lib/hashie/extensions/mash/permissive_respond_to.rb
new file mode 100644
index 0000000..5f8c231
--- /dev/null
+++ b/lib/hashie/extensions/mash/permissive_respond_to.rb
@@ -0,0 +1,61 @@
+module Hashie
+  module Extensions
+    module Mash
+      # Allow a Mash to properly respond to everything
+      #
+      # By default, Mashes only say they respond to methods for keys that exist
+      # in their key set or any of the affix methods (e.g. setter, underbang,
+      # etc.). This causes issues when you try to use them within a
+      # SimpleDelegator or bind to a method for a key that is unset.
+      #
+      # This extension allows a Mash to properly respond to `respond_to?` and
+      # `method` for keys that have not yet been set. This enables full
+      # compatibility with SimpleDelegator and thunk-oriented programming.
+      #
+      # There is a trade-off with this extension: it will run slower than a
+      # regular Mash; insertions and initializations with keys run approximately
+      # 20% slower and cost approximately 19KB of memory per class that you
+      # make permissive.
+      #
+      # @api public
+      # @example Make a new, permissively responding Mash subclass
+      #   class PermissiveMash < Hashie::Mash
+      #     include Hashie::Extensions::Mash::PermissiveRespondTo
+      #   end
+      #
+      #   mash = PermissiveMash.new(a: 1)
+      #   mash.respond_to? :b  #=> true
+      module PermissiveRespondTo
+        # The Ruby hook for behavior when including the module
+        #
+        # @api private
+        # @private
+        # @return void
+        def self.included(base)
+          base.instance_variable_set :@_method_cache, base.instance_methods
+          base.define_singleton_method(:method_cache) { @_method_cache }
+        end
+
+        # The Ruby hook for determining what messages a class might respond to
+        #
+        # @api private
+        # @private
+        def respond_to_missing?(_method_name, _include_private = false)
+          true
+        end
+
+        private
+
+        # Override the Mash logging behavior to account for permissiveness
+        #
+        # @api private
+        # @private
+        def log_collision?(method_key)
+          self.class.method_cache.include?(method_key) &&
+            !self.class.disable_warnings?(method_key) &&
+            !(regular_key?(method_key) || regular_key?(method_key.to_s))
+        end
+      end
+    end
+  end
+end
diff --git a/spec/hashie/extensions/mash/permissive_respond_to_spec.rb b/spec/hashie/extensions/mash/permissive_respond_to_spec.rb
new file mode 100644
index 0000000..189f94a
--- /dev/null
+++ b/spec/hashie/extensions/mash/permissive_respond_to_spec.rb
@@ -0,0 +1,44 @@
+require 'spec_helper'
+
+RSpec.describe Hashie::Extensions::Mash::PermissiveRespondTo do
+  class PermissiveMash < Hashie::Mash
+    include Hashie::Extensions::Mash::PermissiveRespondTo
+  end
+
+  it 'allows you to bind to unset getters' do
+    mash = PermissiveMash.new(a: 1)
+    other_mash = PermissiveMash.new(b: 2)
+
+    expect { mash.method(:b) }.not_to raise_error
+    expect(mash.method(:b).unbind.bind(other_mash).call).to eq 2
+  end
+
+  it 'works properly with SimpleDelegator' do
+    delegator = Class.new(SimpleDelegator) do
+      def initialize(hash)
+        super(PermissiveMash.new(hash))
+      end
+    end
+
+    foo = delegator.new(a: 1)
+
+    expect(foo.a).to eq 1
+    expect { foo.b }.not_to raise_error
+  end
+
+  context 'warnings' do
+    include_context 'with a logger'
+
+    it 'does not log a collision when setting normal keys' do
+      PermissiveMash.new(a: 1)
+
+      expect(logger_output).to be_empty
+    end
+
+    it 'logs a collision with a built-in method' do
+      PermissiveMash.new(zip: 1)
+
+      expect(logger_output).to match('PermissiveMash#zip defined in Enumerable')
+    end
+  end
+end