diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..3bbbd66 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -973,8 +973,11 @@ def data(msgstr = nil, &block)   #:yield: stream
             @socket.write_message_by_block(&block)
           end
         ensure
-          @socket.io.flush
-          @socket.io.sync = socket_sync_bak
+          begin
+            @socket.io.flush
+          ensure
+            @socket.io.sync = socket_sync_bak
+          end
         end
         recv_response()
       }
diff --git a/test/net/smtp/test_smtp.rb b/test/net/smtp/test_smtp.rb
index 3b9e245..8d84fb3 100644
--- a/test/net/smtp/test_smtp.rb
+++ b/test/net/smtp/test_smtp.rb
@@ -253,6 +253,30 @@ def server.data
       assert_equal "250", err.response.status
     end
 
+    def test_data_restores_sync_when_flush_fails
+      io = Object.new
+      class << io
+        attr_accessor :sync
+
+        def flush
+          raise IOError, "flush failed"
+        end
+      end
+      io.sync = true
+      socket = Object.new
+      socket.define_singleton_method(:io) { io }
+      socket.define_singleton_method(:write_message) { |_| }
+
+      smtp = Net::SMTP.new("example.invalid", starttls: false)
+      smtp.instance_variable_set(:@socket, socket)
+      smtp.define_singleton_method(:get_response) do |_|
+        Net::SMTP::Response.parse("354 continue")
+      end
+
+      assert_raise(IOError) { smtp.data("message") }
+      assert_equal true, io.sync
+    end
+
     def test_crlf_injection
       server = FakeServer.new
       smtp = Net::SMTP.new 'localhost', server.port