implement limits on outgoing messages for an account

by default 1000 messages per day, and to max 200 first-time receivers.
i don't think a person would reach those limits. a compromised account abused
by spammers could easily reach that limit. this prevents further damage.

the error message you will get is quite clear, pointing to the configuration
parameter that should be changed.

prometheus.rules

@@ -28,6 +28,16 @@ groups:
     annotations:
       summary: http 5xx responses from webserver
 
+  - alert: mox-submission-errors
+    expr: increase(mox_smtpserver_submission_total{result=~".*error"}[1h]) > 0
+    annotations:
+      summary: smtp submission errors
+
+  - alert: mox-delivery-errors
+    expr: increase(mox_smtpserver_delivery_total{result=~".*error"}[1h]) > 0
+    annotations:
+      summary: smtp delivery errors
+
   # the alerts below can be used to keep a closer eye or when starting to use mox,
   # but can be noisy, or you may not be able to prevent them.