document the recent spampd issue on rude/RT

howto/rt.md

@@ -296,6 +296,31 @@ warnings, the user should just be re-enabled.
 TODO: explain how to block users otherwise. See [issue 33314](https://gitlab.torproject.org/tpo/tpa/team/-/issues/33314) for
 more information.
 
+### Email delivery fails
+
+If no email is coming into RT, it might be one of the spam filters
+that failed to start. Nagios would complain with a message like:
+
+    Subject: ** PROBLEM Service Alert: rude/process - spampd - master is CRITICAL **
+
+    [...]
+
+    Service: process - spampd - master
+    Host: rude
+    Address: 116.202.120.187
+    State: CRITICAL
+
+    Date/Time: Thu Feb 11 14:53:44 UTC 2021
+
+    Additional Info:
+
+    PROCS CRITICAL: 0 processes with UID = 112 (spampd), command name spampd, PPID = 1
+
+It's still unclear why this happens (see e.g. [issue 40175](https://gitlab.torproject.org/tpo/tpa/team/-/issues/40175)), but a
+workaround is to either reboot the server or restart `spampd`:
+
+    systemctl restart spampd
+
 # Reference
 
 ## Installation