The email content can be customized with the notificationsEmailTemplateExtensionPoint. When you create this extension, you can set the custom NotificationTemplateRenderer to the module. To modify the contents, override the getSubject, getHtml and getText methods.

import { notificationsEmailTemplateExtensionPoint } from '@backstage/plugin-notifications-backend-module-email';
import { Notification } from '@backstage/plugin-notifications-common';

export const notificationsModuleEmailDecorator = createBackendModule({
  pluginId: 'notifications',
  moduleId: 'email.templates',
  register(reg) {
    reg.registerInit({
      deps: {
        emailTemplates: notificationsEmailTemplateExtensionPoint,
      },
      async init({ emailTemplates }) {
        emailTemplates.setTemplateRenderer({
          getSubject(notification) {
            return `New notification from ${notification.source}`;
          },
          getText(notification) {
            return notification.content;
          },
          getHtml(notification) {
            return `<p>${notification.content}</p>`;
          },
        });
      },
    });
  },
});

Example configuration:

notifications:
  processors:
    email:
      # Transport config, see options at `config.d.ts`
      transportConfig:
        transport: 'smtp'
        hostname: 'my-smtp-server'
        port: 587
        secure: false
        username: 'my-username'
        password: 'my-password'
      # The email sender address
      sender: 'sender@mycompany.com'
      replyTo: 'no-reply@mycompany.com'
      # Who to get email for broadcast notifications
      broadcastConfig:
        receiver: 'users'
      # How many emails to send concurrently, defaults to 2
      concurrencyLimit: 10
      # Cache configuration for email addresses
      # This is to prevent unnecessary calls to the catalog
      cache:
        ttl:
          days: 1

See config.d.ts for more options for configuration.