diff --git a/lib/net/imap/config.rb b/lib/net/imap/config.rb
index 10740ffb..0b0c028d 100644
--- a/lib/net/imap/config.rb
+++ b/lib/net/imap/config.rb
@@ -54,25 +54,112 @@ class IMAP
     #   plain_client.config.inherited?(:debug)  # => true
     #   plain_client.config.debug?  # => false
     #
+    # == Versioned defaults
+    #
+    # The effective default configuration for a specific +x.y+ version of
+    # +net-imap+ can be loaded with the +config+ keyword argument to
+    # Net::IMAP.new.  Requesting default configurations for previous versions
+    # enables extra backward compatibility with those versions:
+    #
+    #   client = Net::IMAP.new(hostname, config: 0.3)
+    #   client.config.sasl_ir                  # => false
+    #   client.config.responses_without_block  # => :silence_deprecation_warning
+    #
+    #   client = Net::IMAP.new(hostname, config: 0.4)
+    #   client.config.sasl_ir                  # => true
+    #   client.config.responses_without_block  # => :silence_deprecation_warning
+    #
+    #   client = Net::IMAP.new(hostname, config: 0.5)
+    #   client.config.sasl_ir                  # => true
+    #   client.config.responses_without_block  # => :warn
+    #
+    #   client = Net::IMAP.new(hostname, config: :future)
+    #   client.config.sasl_ir                  # => true
+    #   client.config.responses_without_block  # => :raise
+    #
+    # The versioned default configs inherit certain specific config options from
+    # Config.global, for example #debug:
+    #
+    #   client = Net::IMAP.new(hostname, config: 0.4)
+    #   Net::IMAP.debug = false
+    #   client.config.debug?  # => false
+    #
+    #   Net::IMAP.debug = true
+    #   client.config.debug?  # => true
+    #
+    # === Named defaults
+    # In addition to +x.y+ version numbers, the following aliases are supported:
+    #
+    # [+:default+]
+    #   An alias for +:current+.
+    #
+    #   >>>
+    #   *NOTE*: This is _not_ the same as Config.default.  It inherits some
+    #   attributes from Config.global, for example: #debug.
+    # [+:current+]
+    #   An alias for the current +x.y+ version's defaults.
+    # [+:next+]
+    #   The _planned_ config for the next +x.y+ version.
+    # [+:future+]
+    #   The _planned_ eventual config for some future +x.y+ version.
+    #
+    # For example, to raise exceptions for all current deprecations:
+    #   client = Net::IMAP.new(hostname, config: :future)
+    #   client.responses  # raises an ArgumentError
     #
     # == Thread Safety
     #
     # *NOTE:* Updates to config objects are not synchronized for thread-safety.
     #
     class Config
+      # Array of attribute names that are _not_ loaded by #load_defaults.
+      DEFAULT_TO_INHERIT = %i[debug].freeze
+      private_constant :DEFAULT_TO_INHERIT
+
       # The default config, which is hardcoded and frozen.
       def self.default; @default end
 
       # The global config object.  Also available from Net::IMAP.config.
       def self.global; @global end
 
-      def self.[](config) # :nodoc: unfinished API
+      # A hash of hard-coded configurations, indexed by version number.
+      def self.version_defaults; @version_defaults end
+      @version_defaults = {}
+
+      # :call-seq:
+      #  Net::IMAP::Config[number] -> versioned config
+      #  Net::IMAP::Config[symbol] -> named config
+      #  Net::IMAP::Config[hash]   -> new frozen config
+      #  Net::IMAP::Config[config] -> same config
+      #
+      # Given a version number, returns the default configuration for the target
+      # version.  See Config@Versioned+defaults.
+      #
+      # Given a version name, returns the default configuration for the target
+      # version.  See Config@Named+defaults.
+      #
+      # Given a Hash, creates a new _frozen_ config which inherits from
+      # Config.global.  Use Config.new for an unfrozen config.
+      #
+      # Given a config, returns that same config.
+      def self.[](config)
         if config.is_a?(Config) || config.nil? && global.nil?
           config
+        elsif config.respond_to?(:to_hash)
+          new(global, **config).freeze
         else
-          raise TypeError, "no implicit conversion of %s to %s" % [
-            config.class, Config
-          ]
+          version_defaults.fetch(config) {
+            case config
+            when Numeric
+              raise RangeError, "unknown config version: %p" % [config]
+            when Symbol
+              raise KeyError, "unknown config name: %p" % [config]
+            else
+              raise TypeError, "no implicit conversion of %s to %s" % [
+                config.class, Config
+              ]
+            end
+          }
         end
       end
 
@@ -198,6 +285,31 @@ def to_h; data.members.to_h { [_1, send(_1)] } end
 
       @global = default.new
 
+      version_defaults[0.4] = Config[
+        default.to_h.reject {|k,v| DEFAULT_TO_INHERIT.include?(k) }
+      ]
+
+      version_defaults[0] = Config[0.4].dup.update(
+        sasl_ir: false,
+      ).freeze
+      version_defaults[0.0] = Config[0]
+      version_defaults[0.1] = Config[0]
+      version_defaults[0.2] = Config[0]
+      version_defaults[0.3] = Config[0]
+
+      version_defaults[0.5] = Config[0.4].dup.update(
+        responses_without_block: :warn,
+      ).freeze
+
+      version_defaults[:default] = Config[0.4]
+      version_defaults[:current] = Config[0.4]
+      version_defaults[:next]    = Config[0.5]
+
+      version_defaults[:future]  = Config[0.5].dup.update(
+        responses_without_block: :raise,
+      ).freeze
+
+      version_defaults.freeze
     end
   end
 end
diff --git a/test/net/imap/test_config.rb b/test/net/imap/test_config.rb
index 9391eadb..8176f725 100644
--- a/test/net/imap/test_config.rb
+++ b/test/net/imap/test_config.rb
@@ -135,10 +135,64 @@ class ConfigTest < Test::Unit::TestCase
     assert_equal false, child.debug?
   end
 
+  test ".version_defaults are all frozen, and inherit debug from global" do
+    Config.version_defaults.each do |name, config|
+      assert [0, Float, Symbol].any? { _1 === name }
+      assert_kind_of Config, config
+      assert config.frozen?,            "#{name} isn't frozen"
+      assert config.inherited?(:debug), "#{name} doesn't inherit debug"
+      assert_same Config.global, config.parent
+    end
+  end
+
+  test ".[] for all x.y versions" do
+    original = Config[0]
+    assert_kind_of Config, original
+    assert_same original, Config[0.0]
+    assert_same original, Config[0.1]
+    assert_same original, Config[0.2]
+    assert_same original, Config[0.3]
+    assert_kind_of Config, Config[0.4]
+    assert_kind_of Config, Config[0.5]
+  end
+
+  test ".[] range errors" do
+    assert_raise(RangeError) do Config[0.01] end
+    assert_raise(RangeError) do Config[0.11] end
+    assert_raise(RangeError) do Config[0.111] end
+    assert_raise(RangeError) do Config[0.9] end
+    assert_raise(RangeError) do Config[1] end
+  end
+
+  test ".[] key errors" do
+    assert_raise(KeyError) do Config[:nonexistent] end
+  end
+
+  test ".[] with symbol names" do
+    assert_same    Config[0.4],   Config[:current]
+    assert_same    Config[0.4],   Config[:default]
+    assert_same    Config[0.5],   Config[:next]
+    assert_kind_of Config,        Config[:future]
+  end
+
+  test ".[] with a hash" do
+    config = Config[{responses_without_block: :raise, sasl_ir: false}]
+    assert config.frozen?
+    refute config.sasl_ir?
+    assert config.inherited?(:debug)
+    refute config.inherited?(:sasl_ir)
+    assert_same Config.global, config.parent
+    assert_same :raise, config.responses_without_block
+  end
+
   test ".new always sets a parent" do
     assert_same Config.global,  Config.new.parent
     assert_same Config.default, Config.new(Config.default).parent
     assert_same Config.global,  Config.new(Config.global).parent
+    assert_same Config[0.4],    Config.new(0.4).parent
+    assert_same Config[0.5],    Config.new(:next).parent
+    assert_equal true, Config.new({debug: true}, debug: false).parent.debug?
+    assert_equal true, Config.new({debug: true}, debug: false).parent.frozen?
   end
 
   test "#freeze" do