Email providers (Google in most cases) update their security settings from time to time to ensure the most secure experience for their users. When they do, all 3rd party providers and applications are required to 'revalidate' any syncs or integrations.
When this happens, you simply need to refresh SMTP Settings - step-by-step guide below.
In addition, we recommend refreshing your email sync settings (the feature that allows you to see email correspondence without leaving Pixifi) via a similar few steps, you can find the guide here: How to connect your inbox to view email correspondence within Pixifi (aka Email Sync or IMAP)