feat: add possibility to allow and deny specific email addresses

Notification email processor config to support allowing and denying
specific email addresses who can receive notifications in email.
This is required to be able to test this in test environments with real
production data and not send unnecessary notifications.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

.changeset/witty-queens-run.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-notifications-backend-module-email': patch
+---
+
+Notification email processor supports allowing or denying specific email addresses from receiving notifications

plugins/notifications-backend-module-email/config.d.ts

@@ -132,6 +132,14 @@ export interface Config {
            */
           excludedTopics?: string[];
         };
+        /**
+         * White list of addresses to send email to
+         */
+        allowlistEmailAddresses?: string[];
+        /**
+         * Black list of addresses to not send email to
+         */
+        denylistEmailAddresses?: string[];
       };
     };
   };

plugins/notifications-backend-module-email/src/processor/NotificationsEmailProcessor.ts

@@ -56,6 +56,8 @@ export class NotificationsEmailProcessor implements NotificationProcessor {
   private readonly throttleInterval: number;
   private readonly frontendBaseUrl: string;
   private readonly filter: NotificationProcessorFilters;
+  private readonly allowlistEmailAddresses?: string[];
+  private readonly denylistEmailAddresses?: string[];
 
   constructor(
     private readonly logger: LoggerService,
@@ -85,6 +87,12 @@ export class NotificationsEmailProcessor implements NotificationProcessor {
       ? durationToMilliseconds(readDurationFromConfig(cacheConfig))
       : 3_600_000;
     this.frontendBaseUrl = config.getString('app.baseUrl');
+    this.allowlistEmailAddresses = emailProcessorConfig.getOptionalStringArray(
+      'allowlistEmailAddresses',
+    );
+    this.denylistEmailAddresses = emailProcessorConfig.getOptionalStringArray(
+      'denylistEmailAddresses',
+    );
     this.filter = getProcessorFiltersFromConfig(emailProcessorConfig);
   }
 
@@ -197,10 +205,25 @@ export class NotificationsEmailProcessor implements NotificationProcessor {
     notification: Notification,
     options: NotificationSendOptions,
   ) {
+    let emails: string[];
     if (options.recipients.type === 'broadcast' || notification.user === null) {
-      return await this.getBroadcastEmails();
+      emails = await this.getBroadcastEmails();
+    } else {
+      emails = await this.getUserEmail(notification.user);
     }
-    return await this.getUserEmail(notification.user);
+
+    if (this.allowlistEmailAddresses) {
+      emails = emails.filter(email =>
+        this.allowlistEmailAddresses?.includes(email),
+      );
+    }
+
+    if (this.denylistEmailAddresses) {
+      emails = emails.filter(
+        email => !this.denylistEmailAddresses?.includes(email),
+      );
+    }
+    return emails;
   }
 
   private async sendMail(options: Mail.Options) {