diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..cd8af89 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -621,6 +621,7 @@ def started?
     #
     def start(*args, helo: nil, user: nil, secret: nil, password: nil, authtype: nil)
       raise ArgumentError, "wrong number of arguments (given #{args.size}, expected 0..4)" if args.size > 4
+      raise IOError, 'SMTP session already started' if @started
       helo ||= args[0] || 'localhost'
       user ||= args[1]
       secret ||= password || args[2]
@@ -664,7 +665,6 @@ def tcp_socket(address, port)
     end
 
     def do_start(helo_domain, user, secret, authtype)
-      raise IOError, 'SMTP session already started' if @started
       if user || secret || authtype
         check_auth_args authtype, user, secret
       end
diff --git a/test/net/smtp/test_smtp.rb b/test/net/smtp/test_smtp.rb
index 3b9e245..5c0f905 100644
--- a/test/net/smtp/test_smtp.rb
+++ b/test/net/smtp/test_smtp.rb
@@ -530,6 +530,20 @@ def test_start_instance
       smtp.finish
     end
 
+    def test_nested_block_start_preserves_active_session
+      server = FakeServer.start
+      smtp = Net::SMTP.start("localhost", server.port)
+
+      error = assert_raise(IOError) do
+        smtp.start { flunk("nested start must not yield") }
+      end
+      assert_equal "SMTP session already started", error.message
+      assert smtp.started?
+      assert smtp.rset.success?
+    ensure
+      smtp.finish if smtp&.started?
+    end
+
     def test_start_instance_with_position_argument
       port = fake_server_start(auth: 'plain')
       smtp = Net::SMTP.new('localhost', port)