Add denylist for sending emails

This allows us to ignore a testing account with an invalid email
address.

resources/email-denylist.edn

@@ -0,0 +1,6 @@
+;; A set of addresses that we never want to send an email to.
+#{
+  ;; Used by GitHub on the account they use for integration testing token breach
+  ;; reporting, but it doesn't actually exist, resulting in monitoring noise
+  "[email protected]"
+  }

src/clojars/email.clj

@@ -1,34 +1,41 @@
 (ns clojars.email
   (:require
-   [clojars.log :as log])
+   [clojars.log :as log]
+   [clojure.edn :as edn]
+   [clojure.java.io :as io])
   (:import
    (java.util.concurrent
     CountDownLatch
     TimeUnit)
    (org.apache.commons.mail
     SimpleEmail)))
 
+(def ^:private email-denylist
+  (edn/read-string (slurp (io/resource "email-denylist.edn"))))
+
 (defn simple-mailer [{:keys [hostname username password port tls? from]}]
   (fn [to subject message]
     (log/with-context {:tag :email
                        :email-to to
                        :email-subject subject}
       (try
-        (let [mail (doto (SimpleEmail.)
-                     (.setHostName (or hostname "localhost"))
-                     (.setSmtpPort (or port 25))
-                     (.setStartTLSEnabled (boolean tls?))
-                     (.setStartTLSRequired (boolean tls?))
-                     (.setFrom (or from "[email protected]") "Clojars")
-                     (.addTo to)
-                     (.setSubject subject)
-                     (.setMsg message))]
-          (when tls?
-            (.setSslSmtpPort mail (str (or port 25))))
-          (when (and username password)
-            (.setAuthentication mail username password))
-          (.send mail)
-          (log/info {:status :success}))
+        (if (contains? email-denylist to)
+          (log/info {:status :denylist})
+          (let [mail (doto (SimpleEmail.)
+                       (.setHostName (or hostname "localhost"))
+                       (.setSmtpPort (or port 25))
+                       (.setStartTLSEnabled (boolean tls?))
+                       (.setStartTLSRequired (boolean tls?))
+                       (.setFrom (or from "[email protected]") "Clojars")
+                       (.addTo to)
+                       (.setSubject subject)
+                       (.setMsg message))]
+            (when tls?
+              (.setSslSmtpPort mail (str (or port 25))))
+            (when (and username password)
+              (.setAuthentication mail username password))
+            (.send mail)
+            (log/info {:status :success})))
         (catch Exception e
           (log/error {:status :failed
                       :error e})