fix: Send SMTP emails sequentially

api/email/post.go

@@ -47,6 +47,8 @@ func HandlePostEmail(
 	sourceEmailPassword := getEnv("SOURCE_EMAIL_PASSWORD")
 	skipTlsVerify := getEnv("TEST_ONLY_SKIP_TLS_VERIFY") == "dummy string just in case"
 
+	smtpMessageMutex := sync.Mutex{}
+
 	buildMessage := func(email *requestBody) string {
 		return fmt.Sprintf(
 			"To: %s\r\nSubject: %s\r\n\r\n%s\r\n\r\nSent by %s",
@@ -102,6 +104,8 @@ func HandlePostEmail(
 
 	sendEmail := func(request *http.Request, client *smtp.Client, email *requestBody) (err error) {
 		doneChannel := make(chan struct{})
+		smtpMessageMutex.Lock()
+		defer smtpMessageMutex.Unlock()
 
 		log.Println("[DEBUG] Setting SMTP email sender")
 		go func() {

api/email/post_test.go

@@ -145,6 +145,33 @@ func TestCancellation(t *testing.T) {
 	assert.Equal(t, expectedErrorRedirectUrl, response.Header.Get("Location"))
 }
 
+func TestConcurrentRequests(t *testing.T) {
+	concurrentRequests := 10
+	emailsReceived := 0
+	smtpHandler := func(_ net.Addr, _ string, _ []string, _ []byte) error {
+		emailsReceived++
+		return nil
+	}
+
+	smtpServer, smtpServerPort := setupSmtpServer(t, smtpHandler, nil)
+	defer teardownSmtpServer(smtpServer)
+	testHttpServer, shutdownWaitGroup, triggerShutdown := setupHttpServer(context.Background(), smtpServerPort)
+	defer teardownHttpServer(testHttpServer, shutdownWaitGroup, triggerShutdown)
+
+	httpRequestCompleted := make(chan struct{})
+	for range concurrentRequests {
+		go func() {
+			requestPostEmail(t, testHttpServer.URL)
+			httpRequestCompleted <- struct{}{}
+		}()
+	}
+
+	for range concurrentRequests {
+		<-httpRequestCompleted
+	}
+	assert.Equal(t, concurrentRequests, emailsReceived)
+}
+
 func setupSmtpServer(t *testing.T, handler smtpd.Handler, authHandler smtpd.AuthHandler) (*smtpd.Server, int) {
 	smtpServer := newSmtpServer(t, handler, authHandler)
 	smtpListener, smtpServerPort := newSmtpServerListener()