diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index 19baff6..ce4b23e 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -704,7 +704,9 @@ def tcp_socket(address, port)
 
     def do_start(helo_domain, user, secret, authtype, **auth)
       raise IOError, 'SMTP session already started' if @started
-      check_auth_args(authtype, user, secret, **auth)
+      if user || secret || authtype || auth.any?
+        check_auth_args(authtype, user, secret, **auth)
+      end
       s = Timeout.timeout(@open_timeout, Net::OpenTimeout) do
         tcp_socket(@address, @port)
       end
@@ -946,8 +948,8 @@ def auth(authtype = DEFAULT_AUTH_TYPE, *args, **kwargs, &block)
 
     private
 
-    def check_auth_args(type_arg = nil, *args, type: nil, **kwargs)
-      return unless type_arg || type || args.any? || kwargs.any?
+    def check_auth_args(type_arg = nil, *args,
+                        type: nil, user: nil, **kwargs)
       type ||= type_arg || DEFAULT_AUTH_TYPE
       kwargs[:username] ||= user if user
       if (klass = Authenticator.auth_class(type))