diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index 9b8ff4e..e962c04 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -31,6 +31,17 @@ module Net
   module SMTPError
     # This *class* is a module for backward compatibility.
     # In later release, this module becomes a class.
+
+    attr_reader :response
+
+    def initialize(response, message: nil)
+      @response = response
+      @message = message
+    end
+
+    def message
+      @message || response.message
+    end
   end
 
   # Represents an SMTP authentication error.
@@ -609,8 +620,7 @@ def do_start(helo_domain, user, secret, authtype)
       do_helo helo_domain
       if ! tls? and (starttls_always? or (capable_starttls? and starttls_auto?))
         unless capable_starttls?
-          raise SMTPUnsupportedCommand,
-              "STARTTLS is not supported on this server"
+          raise SMTPUnsupportedCommand.new(nil, message: "STARTTLS is not supported on this server")
         end
         starttls
         @socket = new_internet_message_io(tlsconnect(s, @ssl_context_starttls))
@@ -1013,25 +1023,25 @@ def critical
 
     def check_response(res)
       unless res.success?
-        raise res.exception_class, res.message
+        raise res.exception_class.new(res)
       end
     end
 
     def check_continue(res)
       unless res.continue?
-        raise SMTPUnknownError, "could not get 3xx (#{res.status}: #{res.string})"
+        raise SMTPUnknownError.new(res, message: "could not get 3xx (#{res.status}: #{res.string})")
       end
     end
 
     def check_auth_response(res)
       unless res.success?
-        raise SMTPAuthenticationError, res.message
+        raise SMTPAuthenticationError.new(res)
       end
     end
 
     def check_auth_continue(res)
       unless res.continue?
-        raise res.exception_class, res.message
+        raise res.exception_class.new(res)
       end
     end
 
diff --git a/test/net/smtp/test_smtp.rb b/test/net/smtp/test_smtp.rb
index 8a5f8db..dc1d962 100644
--- a/test/net/smtp/test_smtp.rb
+++ b/test/net/smtp/test_smtp.rb
@@ -101,6 +101,93 @@ def test_auth_plain
       assert_equal "AUTH PLAIN AGZvbwBiYXI=\r\n", sock.write_io.string
     end
 
+    def test_unsucessful_auth_plain
+      sock = FakeSocket.new("535 Authentication failed: FAIL\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPAuthenticationError) { smtp.auth_plain("foo", "bar") }
+      assert_equal "535 Authentication failed: FAIL\n", err.message
+      assert_equal "535", err.response.status
+    end
+
+    def test_auth_login
+      sock = FakeSocket.new("334 VXNlcm5hbWU6\r\n334 UGFzc3dvcmQ6\r\n235 2.7.0 Authentication successful\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      assert smtp.auth_login("foo", "bar").success?
+    end
+
+    def test_unsucessful_auth_login
+      sock = FakeSocket.new("334 VXNlcm5hbWU6\r\n334 UGFzc3dvcmQ6\r\n535 Authentication failed: FAIL\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPAuthenticationError) { smtp.auth_login("foo", "bar") }
+      assert_equal "535 Authentication failed: FAIL\n", err.message
+      assert_equal "535", err.response.status
+    end
+
+    def test_non_continue_auth_login
+      sock = FakeSocket.new("334 VXNlcm5hbWU6\r\n235 2.7.0 Authentication successful\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPUnknownError) { smtp.auth_login("foo", "bar") }
+      assert_equal "235 2.7.0 Authentication successful\n", err.message
+      assert_equal "235", err.response.status
+    end
+
+    def test_unsuccessful_send_message_server_busy
+      sock = FakeSocket.new("400 BUSY\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPServerBusy) { smtp.send_message('message', 'ojab@example.com') }
+      assert_equal "400 BUSY\n", err.message
+      assert_equal "400", err.response.status
+    end
+
+    def test_unsuccessful_send_message_syntax_error
+      sock = FakeSocket.new("502 SYNTAX ERROR\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPSyntaxError) { smtp.send_message('message', 'ojab@example.com') }
+      assert_equal "502 SYNTAX ERROR\n", err.message
+      assert_equal "502", err.response.status
+    end
+
+    def test_unsuccessful_send_message_authentication_error
+      sock = FakeSocket.new("530 AUTH ERROR\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPAuthenticationError) { smtp.send_message('message', 'ojab@example.com') }
+      assert_equal "530 AUTH ERROR\n", err.message
+      assert_equal "530", err.response.status
+    end
+
+    def test_unsuccessful_send_message_fatal_error
+      sock = FakeSocket.new("520 FATAL ERROR\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPFatalError) { smtp.send_message('message', 'ojab@example.com') }
+      assert_equal "520 FATAL ERROR\n", err.message
+      assert_equal "520", err.response.status
+    end
+
+    def test_unsuccessful_send_message_unknown_error
+      sock = FakeSocket.new("300 UNKNOWN\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPUnknownError) { smtp.send_message('message', 'ojab@example.com') }
+      assert_equal "300 UNKNOWN\n", err.message
+      assert_equal "300", err.response.status
+    end
+
+    def test_unsuccessful_data
+      sock = FakeSocket.new("250 OK\r\n")
+      smtp = Net::SMTP.new 'localhost', 25
+      smtp.instance_variable_set :@socket, sock
+      err = assert_raise(Net::SMTPUnknownError) { smtp.data('message') }
+      assert_equal "could not get 3xx (250: 250 OK\n)", err.response.status
+    end
+
     def test_crlf_injection
       smtp = Net::SMTP.new 'localhost', 25
       smtp.instance_variable_set :@socket, FakeSocket.new
diff --git a/test/net/smtp/test_starttls.rb b/test/net/smtp/test_starttls.rb
index 096eaad..f2bb201 100644
--- a/test/net/smtp/test_starttls.rb
+++ b/test/net/smtp/test_starttls.rb
@@ -76,6 +76,7 @@ def test_enable_starttls_without_starttls_capable
       smtp.enable_starttls
       err = assert_raise(Net::SMTPUnsupportedCommand) { smtp.start }
       assert_equal("STARTTLS is not supported on this server", err.message)
+      assert_nil(err.response)
     end
 
     def test_enable_starttls_auto_with_starttls_capable