Add missing error key to sample config

config.toml.sample

@@ -26,6 +26,11 @@ asset_path = "frontend/my/build"
 # mail server will  
 concurrency = 100
 
+# The number of errors (eg: SMTP timeouts while e-mailing) a running
+# campaign should tolerate before it is paused for manual
+# investigation or intervention. Set to 0 to never pause.
+max_send_errors = 10
+
 
 # Database.
 [db]