diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index 86b55d278b..56600a077a 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -38,7 +38,7 @@ module Net
     include SMTPError
   end
 
-  # Represents SMTP error code 420 or 450, a temporary error.
+  # Represents SMTP error code 4xx, a temporary error.
   class SMTPServerBusy < ProtoServerError
     include SMTPError
   end