diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 96ec3a50..232b3728 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -725,6 +725,8 @@ class IMAP < Protocol
       "UTF8=ONLY" => "UTF8=ACCEPT",
     }.freeze
 
+    autoload :Config,      File.expand_path("imap/config",       __dir__)
+
     autoload :SASL,        File.expand_path("imap/sasl",         __dir__)
     autoload :SASLAdapter, File.expand_path("imap/sasl_adapter", __dir__)
     autoload :StringPrep,  File.expand_path("imap/stringprep",   __dir__)
@@ -735,14 +737,15 @@ class IMAP < Protocol
       include SSL
     end
 
-    # Returns the debug mode.
-    def self.debug
-      return @@debug
-    end
+    # Returns the global Config object
+    def self.config; Config.global end
 
-    # Sets the debug mode.
+    # Returns the global debug mode.
+    def self.debug; config.debug end
+
+    # Sets the global debug mode.
     def self.debug=(val)
-      return @@debug = val
+      config.debug = val
     end
 
     # The default port for IMAP connections, port 143
@@ -764,13 +767,18 @@ class << self
     # Returns the initial greeting the server, an UntaggedResponse.
     attr_reader :greeting
 
+    # The client configuration.  See Net::IMAP::Config.
+    #
+    # By default, config inherits from the global Net::IMAP.config.
+    attr_reader :config
+
     # Seconds to wait until a connection is opened.
     # If the IMAP object cannot open a connection within this time,
     # it raises a Net::OpenTimeout exception. The default value is 30 seconds.
-    attr_reader :open_timeout
+    def open_timeout; config.open_timeout end
 
     # Seconds to wait until an IDLE response is received.
-    attr_reader :idle_response_timeout
+    def idle_response_timeout; config.idle_response_timeout end
 
     # The hostname this client connected to
     attr_reader :host
@@ -811,6 +819,15 @@ class << self
     #   the keys are names of attribute assignment methods on
     #   SSLContext[https://docs.ruby-lang.org/en/master/OpenSSL/SSL/SSLContext.html].
     #
+    # [config]
+    #   A Net::IMAP::Config object to base the client #config on.  By default
+    #   the global Net::IMAP.config is used.  Note that this sets the _parent_
+    #   config object for inheritance.  Every Net::IMAP client has its own
+    #   unique #config for overrides.
+    #
+    # Any other keyword arguments will be forwarded to Config.new, to create the
+    # client's #config.  For example:
+    #
     # [open_timeout]
     #   Seconds to wait until a connection is opened
     # [idle_response_timeout]
@@ -872,13 +889,12 @@ class << self
     #   Connected to the host successfully, but it immediately said goodbye.
     #
     def initialize(host, port: nil, ssl:  nil,
-                   open_timeout: 30, idle_response_timeout: 5)
+                   config: Config.global, **config_options)
       super()
       # Config options
       @host = host
+      @config = Config.new(config, **config_options)
       @port = port || (ssl ? SSL_PORT : PORT)
-      @open_timeout = Integer(open_timeout)
-      @idle_response_timeout = Integer(idle_response_timeout)
       @ssl_ctx_params, @ssl_ctx = build_ssl_ctx(ssl)
 
       # Basic Client State
@@ -889,7 +905,7 @@ def initialize(host, port: nil, ssl:  nil,
       @capabilities = nil
 
       # Client Protocol Receiver
-      @parser = ResponseParser.new
+      @parser = ResponseParser.new(config: @config)
       @responses = Hash.new {|h, k| h[k] = [] }
       @response_handlers = []
       @receiver_thread = nil
@@ -2434,7 +2450,7 @@ def idle(timeout = nil, &response_handler)
           unless @receiver_thread_terminating
             remove_response_handler(response_handler)
             put_string("DONE#{CRLF}")
-            response = get_tagged_response(tag, "IDLE", @idle_response_timeout)
+            response = get_tagged_response(tag, "IDLE", idle_response_timeout)
           end
         end
       end
@@ -2590,8 +2606,6 @@ def remove_response_handler(handler)
     PORT = 143         # :nodoc:
     SSL_PORT = 993   # :nodoc:
 
-    @@debug = false
-
     def start_imap_connection
       @greeting        = get_server_greeting
       @capabilities    = capabilities_from_resp_code @greeting
@@ -2619,12 +2633,12 @@ def start_receiver_thread
     end
 
     def tcp_socket(host, port)
-      s = Socket.tcp(host, port, :connect_timeout => @open_timeout)
+      s = Socket.tcp(host, port, :connect_timeout => open_timeout)
       s.setsockopt(:SOL_SOCKET, :SO_KEEPALIVE, true)
       s
     rescue Errno::ETIMEDOUT
       raise Net::OpenTimeout, "Timeout to open TCP connection to " +
-        "#{host}:#{port} (exceeds #{@open_timeout} seconds)"
+        "#{host}:#{port} (exceeds #{open_timeout} seconds)"
     end
 
     def receive_responses
@@ -2736,7 +2750,7 @@ def get_response
         end
       end
       return nil if buff.length == 0
-      if @@debug
+      if config.debug?
         $stderr.print(buff.gsub(/^/n, "S: "))
       end
       return @parser.parse(buff)
@@ -2815,7 +2829,7 @@ def generate_tag
 
     def put_string(str)
       @sock.print(str)
-      if @@debug
+      if config.debug?
         if @debug_output_bol
           $stderr.print("C: ")
         end
@@ -2942,7 +2956,7 @@ def start_tls_session
       @sock = SSLSocket.new(@sock, ssl_ctx)
       @sock.sync_close = true
       @sock.hostname = @host if @sock.respond_to? :hostname=
-      ssl_socket_connect(@sock, @open_timeout)
+      ssl_socket_connect(@sock, open_timeout)
       if ssl_ctx.verify_mode != VERIFY_NONE
         @sock.post_connection_check(@host)
         @tls_verified = true
diff --git a/lib/net/imap/config.rb b/lib/net/imap/config.rb
new file mode 100644
index 00000000..f8201c5c
--- /dev/null
+++ b/lib/net/imap/config.rb
@@ -0,0 +1,109 @@
+# frozen_string_literal: true
+# :markup: markdown
+
+require_relative "config/attr_accessors"
+require_relative "config/attr_inheritance"
+require_relative "config/attr_type_coercion"
+
+module Net
+  class IMAP
+
+    # Net::IMAP::Config stores configuration options for Net::IMAP clients.
+    # The global configuration can be seen at either Net::IMAP.config or
+    # Net::IMAP::Config.global, and the client-specific configuration can be
+    # seen at Net::IMAP#config.  When creating a new client, all unhandled
+    # keyword arguments to Net::IMAP.new are delegated to Config.new.  Every
+    # client has its own config.
+    #
+    # ## Inheritance
+    #
+    # Configs have a parent[rdoc-ref:Config::AttrInheritance#parent] config, and
+    # any attributes which have not been set locally will inherit the parent's
+    # value.  Every client creates its own specific config.  By default, client
+    # configs inherit from Config.global which inherits from Config.default.
+    #
+    # See the following methods, defined by Config::AttrInheritance:
+    # - {#new}[rdoc-ref:Config::AttrInheritance#reset] -- create a new config
+    #   which inherits from the receiver.
+    # - {#inherited?}[rdoc-ref:Config::AttrInheritance#inherited?] -- return
+    #   whether a particular attribute is inherited.
+    # - {#reset}[rdoc-ref:Config::AttrInheritance#reset] -- reset attributes to
+    #   be inherited.
+    #
+    # ## Thread Safety
+    #
+    # *NOTE:* Updates to config objects are not synchronized for thread-safety.
+    #
+    class Config
+      # The default config, which is hardcoded and frozen.
+      def self.default; @default end
+
+      # The global config object.
+      def self.global; @global end
+
+      def self.[](config) # :nodoc: unfinished API
+        if config.is_a?(Config) || config.nil? && global.nil?
+          config
+        else
+          raise TypeError, "no implicit conversion of %s to %s" % [
+            config.class, Config
+          ]
+        end
+      end
+
+      include AttrAccessors
+      include AttrInheritance
+      include AttrTypeCoercion
+
+      # The debug mode (boolean)
+      #
+      # | Starting with version | The default value is |
+      # |-----------------------|----------------------|
+      # | _original_            | +false+              |
+      attr_accessor :debug, type: :boolean
+
+      # method: debug?
+      # :call-seq: debug? -> boolean
+      #
+      # Alias for #debug
+
+      # Seconds to wait until a connection is opened.
+      #
+      # If the IMAP object cannot open a connection within this time,
+      # it raises a Net::OpenTimeout exception.  See Net::IMAP.new.
+      #
+      # | Starting with version | The default value is |
+      # |-----------------------|----------------------|
+      # | _original_            | +30+ seconds         |
+      attr_accessor :open_timeout, type: Integer
+
+      # Seconds to wait until an IDLE response is received, after
+      # the client asks to leave the IDLE state.  See Net::IMAP#idle_done.
+      #
+      # | Starting with version | The default value is |
+      # |-----------------------|----------------------|
+      # | _original_            | +5+ seconds          |
+      attr_accessor :idle_response_timeout, type: Integer
+
+      # Creates a new config object and initialize its attribute with +attrs+.
+      #
+      # If +parent+ is not given, the global config is used by default.
+      #
+      # If a block is given, the new config object is yielded to it.
+      def initialize(parent = Config.global, **attrs)
+        super(parent)
+        attrs.each do send(:"#{_1}=", _2) end
+        yield self if block_given?
+      end
+
+      @default = new(
+        debug: false,
+        open_timeout: 30,
+        idle_response_timeout: 5,
+      ).freeze
+
+      @global = default.new
+
+    end
+  end
+end
diff --git a/lib/net/imap/config/attr_accessors.rb b/lib/net/imap/config/attr_accessors.rb
new file mode 100644
index 00000000..2cb2272c
--- /dev/null
+++ b/lib/net/imap/config/attr_accessors.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+require "forwardable"
+
+module Net
+  class IMAP
+    class Config
+
+      # Config values are stored in a struct rather than ivars to simplify:
+      # * ensuring that all config objects share a single object shape
+      # * querying only locally configured values, e.g for inspection.
+      module AttrAccessors
+        module Macros # :nodoc: internal API
+          def attr_accessor(name) AttrAccessors.attr_accessor(name) end
+        end
+        private_constant :Macros
+
+        def self.included(mod)
+          mod.extend Macros
+        end
+        private_class_method :included
+
+        extend Forwardable
+
+        def self.attr_accessor(name) # :nodoc: internal API
+          name = name.to_sym
+          def_delegators :data, name, :"#{name}="
+        end
+
+        def self.attributes
+          instance_methods.grep(/=\z/).map { _1.to_s.delete_suffix("=").to_sym }
+        end
+        private_class_method :attributes
+
+        def self.struct # :nodoc: internal API
+          unless defined?(self::Struct)
+            const_set :Struct, Struct.new(*attributes)
+          end
+          self::Struct
+        end
+
+        def initialize # :notnew:
+          super()
+          @data = AttrAccessors.struct.new
+        end
+
+        # Freezes the internal attributes struct, in addition to +self+.
+        def freeze
+          data.freeze
+          super
+        end
+
+        protected
+
+        attr_reader :data # :nodoc: internal API
+
+        private
+
+        def initialize_dup(other)
+          super
+          @data = other.data.dup
+        end
+
+      end
+    end
+  end
+end
diff --git a/lib/net/imap/config/attr_inheritance.rb b/lib/net/imap/config/attr_inheritance.rb
new file mode 100644
index 00000000..be68a8cb
--- /dev/null
+++ b/lib/net/imap/config/attr_inheritance.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP
+    class Config
+      # Inheritance forms a singly-linked-list, so lookup will be O(n) on the
+      # number of ancestors.  Without customization, ancestor trees will only be
+      # three or four deep:
+      #     client -> [versioned ->] global -> default
+      module AttrInheritance
+        INHERITED = Module.new.freeze
+        private_constant :INHERITED
+
+        module Macros # :nodoc: internal API
+          def attr_accessor(name) super; AttrInheritance.attr_accessor(name) end
+        end
+        private_constant :Macros
+
+        def self.included(mod)
+          mod.extend Macros
+        end
+        private_class_method :included
+
+        def self.attr_accessor(name) # :nodoc: internal API
+          module_eval <<~RUBY, __FILE__, __LINE__ + 1
+            def #{name}; (val = super) == INHERITED ? parent&.#{name} : val end
+          RUBY
+        end
+
+        # The parent Config object
+        attr_reader :parent
+
+        def initialize(parent = nil) # :notnew:
+          super()
+          @parent = Config[parent]
+          reset
+        end
+
+        # Creates a new config, which inherits from +self+.
+        def new(**attrs) self.class.new(self, **attrs) end
+
+        # Returns +true+ if +attr+ is inherited from #parent and not overridden
+        # by this config.
+        def inherited?(attr) data[attr] == INHERITED end
+
+        # :call-seq:
+        #   reset -> self
+        #   reset(attr) -> attribute value
+        #
+        # Resets an +attr+ to inherit from the #parent config.
+        #
+        # When +attr+ is nil or not given, all attributes are reset.
+        def reset(attr = nil)
+          if attr.nil?
+            data.members.each do |attr| data[attr] = INHERITED end
+            self
+          elsif inherited?(attr)
+            nil
+          else
+            old, data[attr] = data[attr], INHERITED
+            old
+          end
+        end
+
+        private
+
+        def initialize_copy(other)
+          super
+          @parent ||= other # only default has nil parent
+        end
+
+      end
+    end
+  end
+end
diff --git a/lib/net/imap/config/attr_type_coercion.rb b/lib/net/imap/config/attr_type_coercion.rb
new file mode 100644
index 00000000..7511193d
--- /dev/null
+++ b/lib/net/imap/config/attr_type_coercion.rb
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP
+    class Config
+      # Adds a +type+ keyword parameter to +attr_accessor+, which enforces
+      # config attributes have valid types, for example: boolean, numeric,
+      # enumeration, non-nullable, etc.
+      module AttrTypeCoercion
+        # :stopdoc: internal APIs only
+
+        module Macros # :nodoc: internal API
+          def attr_accessor(attr, type: nil)
+            super(attr)
+            AttrTypeCoercion.attr_accessor(attr, type: type)
+          end
+        end
+        private_constant :Macros
+
+        def self.included(mod)
+          mod.extend Macros
+        end
+        private_class_method :included
+
+        def self.attr_accessor(attr, type: nil)
+          return unless type
+          if    :boolean == type then boolean attr
+          elsif Integer  == type then integer attr
+          else raise ArgumentError, "unknown type coercion %p" % [type]
+          end
+        end
+
+        def self.boolean(attr)
+          define_method :"#{attr}=" do |val| super !!val end
+          define_method :"#{attr}?" do send attr end
+        end
+
+        def self.integer(attr)
+          define_method :"#{attr}=" do |val| super Integer val end
+        end
+
+      end
+    end
+  end
+end
diff --git a/lib/net/imap/response_parser.rb b/lib/net/imap/response_parser.rb
index 84f6cf79..fe6badc8 100644
--- a/lib/net/imap/response_parser.rb
+++ b/lib/net/imap/response_parser.rb
@@ -11,12 +11,15 @@ class ResponseParser
       include ParserUtils
       extend  ParserUtils::Generator
 
+      attr_reader :config
+
       # :call-seq: Net::IMAP::ResponseParser.new -> Net::IMAP::ResponseParser
-      def initialize
+      def initialize(config: Config.global)
         @str = nil
         @pos = nil
         @lex_state = nil
         @token = nil
+        @config = Config[config]
       end
 
       # :call-seq:
diff --git a/lib/net/imap/response_parser/parser_utils.rb b/lib/net/imap/response_parser/parser_utils.rb
index d458ede7..bc84bed9 100644
--- a/lib/net/imap/response_parser/parser_utils.rb
+++ b/lib/net/imap/response_parser/parser_utils.rb
@@ -154,7 +154,7 @@ def accept(*args)
         end
 
         # To be used conditionally:
-        #   assert_no_lookahead if Net::IMAP.debug
+        #   assert_no_lookahead if config.debug?
         def assert_no_lookahead
           @token.nil? or
             parse_error("assertion failed: expected @token.nil?, actual %s: %p",
@@ -181,23 +181,23 @@ def lookahead!(*args)
         end
 
         def peek_str?(str)
-          assert_no_lookahead if Net::IMAP.debug
+          assert_no_lookahead if config.debug?
           @str[@pos, str.length] == str
         end
 
         def peek_re(re)
-          assert_no_lookahead if Net::IMAP.debug
+          assert_no_lookahead if config.debug?
           re.match(@str, @pos)
         end
 
         def accept_re(re)
-          assert_no_lookahead if Net::IMAP.debug
+          assert_no_lookahead if config.debug?
           re.match(@str, @pos) and @pos = $~.end(0)
           $~
         end
 
         def match_re(re, name)
-          assert_no_lookahead if Net::IMAP.debug
+          assert_no_lookahead if config.debug?
           if re.match(@str, @pos)
             @pos = $~.end(0)
             $~
@@ -212,7 +212,7 @@ def shift_token
 
         def parse_error(fmt, *args)
           msg = format(fmt, *args)
-          if IMAP.debug
+          if config.debug?
             local_path = File.dirname(__dir__)
             tok = @token ? "%s: %p" % [@token.symbol, @token.value] : "nil"
             warn "%s %s: %s"        % [self.class, __method__, msg]
diff --git a/test/net/imap/test_config.rb b/test/net/imap/test_config.rb
new file mode 100644
index 00000000..e14d7b74
--- /dev/null
+++ b/test/net/imap/test_config.rb
@@ -0,0 +1,155 @@
+# frozen_string_literal: true
+
+require "net/imap"
+require "test/unit"
+
+class ConfigTest < Test::Unit::TestCase
+  Config = Net::IMAP::Config
+
+  setup do
+    Config.global.reset
+  end
+
+  test "#debug" do
+    assert Config.new(debug: true).debug
+    refute Config.new(debug: false).debug
+    assert Config.new(debug: true).debug?
+    refute Config.new(debug: false).debug?
+    config = Config.new do |c|
+      c.debug = true
+    end
+    assert config.debug
+    config = Config.new
+    config.debug = true
+    assert config.debug
+    assert config.debug?
+    config.debug = false
+    refute config.debug
+    refute config.debug?
+  end
+
+  test "boolean type constraints and conversion" do
+    config = Config.new
+    config.debug = 111
+    assert_equal true, config.debug
+    config.debug = nil
+    assert_equal false, config.debug
+  end
+
+  test "integer type constraints and conversion" do
+    config = Config.new
+    config.open_timeout = "111"
+    assert_equal 111, config.open_timeout
+    config.open_timeout = 222.0
+    assert_equal 222, config.open_timeout
+    config.open_timeout = 333.3
+    assert_equal 333, config.open_timeout
+    assert_raise(ArgumentError) do
+      config.open_timeout = "444 NaN"
+    end
+    assert_equal 333, config.open_timeout
+  end
+
+  test ".default" do
+    default = Config.default
+    assert default.equal?(Config.default)
+    assert default.is_a?(Config)
+    assert default.frozen?
+    refute default.debug?
+  end
+
+  test ".global" do
+    global = Config.global
+    assert global.equal?(Config.global)
+    assert global.is_a?(Config)
+    assert_same Config.default, global.parent
+    assert_equal false, global.debug?
+    global.debug = true
+    assert_equal true, global.debug?
+    global.reset(:debug)
+    assert_equal false, global.debug?
+    refute global.frozen?
+  end
+
+  test "Net::IMAP.config" do
+    assert Net::IMAP.config.equal?(Config.global)
+  end
+
+  test ".new(parent, ...) and inheritance" do
+    base = Config.new debug: false
+    child = Config.new(base)
+    assert_equal base, child.parent
+    assert_equal false, child.debug
+    assert_equal false, child.debug?
+    base.debug = true
+    assert_equal true, child.debug?
+    child.debug = false
+    assert_equal false, child.debug?
+    child.reset(:debug)
+    assert_equal true, child.debug?
+    base.debug = false
+    child.debug = true
+    assert_equal true, child.debug?
+    child = Config.new(base, debug: true)
+    assert_equal true, child.debug?
+    base.debug = true
+    child = Config.new(base, debug: false)
+    assert_equal false, child.debug?
+  end
+
+  test "#new and inheritance" do
+    base = Config.new debug: false
+    child = base.new
+    assert_equal base, child.parent
+    assert_equal false, child.debug
+    assert_equal false, child.debug?
+    base.debug = true
+    assert_equal true, child.debug?
+    child.debug = false
+    assert_equal false, child.debug?
+    child.reset(:debug)
+    assert_equal true, child.debug?
+    base.debug = false
+    child.debug = true
+    assert_equal true, child.debug?
+    child = base.new(debug: true)
+    assert_equal true, child.debug?
+    base.debug = true
+    child = base.new(debug: false)
+    assert_equal false, child.debug?
+  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
+  end
+
+  test "#inherited? and #reset(attr)" do
+    base = Config.new debug: false, open_timeout: 99, idle_response_timeout: 15
+    child = base.new debug: true, open_timeout: 15, idle_response_timeout: 10
+    refute child.inherited?(:idle_response_timeout)
+    assert_equal 10, child.reset(:idle_response_timeout)
+    assert child.inherited?(:idle_response_timeout)
+    assert_equal 15, child.idle_response_timeout
+    refute child.inherited?(:open_timeout)
+    refute child.inherited?(:debug)
+    child.debug = false
+    refute child.inherited?(:debug)
+    assert_equal false, child.reset(:debug)
+    assert child.inherited?(:debug)
+    assert_equal false, child.debug
+    assert_equal nil, child.reset(:debug)
+  end
+
+  test "#reset all attributes" do
+    base = Config.new debug: false, open_timeout: 99, idle_response_timeout: 15
+    child = base.new debug: true, open_timeout: 15, idle_response_timeout: 10
+    result = child.reset
+    assert_same child, result
+    assert child.inherited?(:debug)
+    assert child.inherited?(:open_timeout)
+    assert child.inherited?(:idle_response_timeout)
+  end
+
+end
diff --git a/test/net/imap/test_deprecated_client_options.rb b/test/net/imap/test_deprecated_client_options.rb
index 1dcdc339..042bf90f 100644
--- a/test/net/imap/test_deprecated_client_options.rb
+++ b/test/net/imap/test_deprecated_client_options.rb
@@ -8,6 +8,7 @@ class DeprecatedClientOptionsTest < Test::Unit::TestCase
   include Net::IMAP::FakeServer::TestHelper
 
   def setup
+    Net::IMAP.config.reset
     @do_not_reverse_lookup = Socket.do_not_reverse_lookup
     Socket.do_not_reverse_lookup = true
     @threads = []
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index 0078956d..30012666 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -12,6 +12,7 @@ class IMAPTest < Test::Unit::TestCase
   include Net::IMAP::FakeServer::TestHelper
 
   def setup
+    Net::IMAP.config.reset
     @do_not_reverse_lookup = Socket.do_not_reverse_lookup
     Socket.do_not_reverse_lookup = true
     @threads = []
diff --git a/test/net/imap/test_imap_capabilities.rb b/test/net/imap/test_imap_capabilities.rb
index 14e1f79b..252a7d9b 100644
--- a/test/net/imap/test_imap_capabilities.rb
+++ b/test/net/imap/test_imap_capabilities.rb
@@ -9,6 +9,7 @@ class IMAPCapabilitiesTest < Test::Unit::TestCase
   include Net::IMAP::FakeServer::TestHelper
 
   def setup
+    Net::IMAP.config.reset
     @do_not_reverse_lookup = Socket.do_not_reverse_lookup
     Socket.do_not_reverse_lookup = true
     @threads = []
diff --git a/test/net/imap/test_imap_response_data.rb b/test/net/imap/test_imap_response_data.rb
index dc546432..0bee8b9a 100644
--- a/test/net/imap/test_imap_response_data.rb
+++ b/test/net/imap/test_imap_response_data.rb
@@ -6,6 +6,7 @@
 class IMAPResponseDataTest < Test::Unit::TestCase
 
   def setup
+    Net::IMAP.config.reset
     @do_not_reverse_lookup = Socket.do_not_reverse_lookup
     Socket.do_not_reverse_lookup = true
   end
diff --git a/test/net/imap/test_imap_response_parser.rb b/test/net/imap/test_imap_response_parser.rb
index 0d4682cd..a4d70f92 100644
--- a/test/net/imap/test_imap_response_parser.rb
+++ b/test/net/imap/test_imap_response_parser.rb
@@ -11,6 +11,7 @@ class IMAPResponseParserTest < Test::Unit::TestCase
   extend  NetIMAPTestHelpers::TestFixtureGenerators
 
   def setup
+    Net::IMAP.config.reset
     @do_not_reverse_lookup = Socket.do_not_reverse_lookup
     Socket.do_not_reverse_lookup = true
   end