The GP team has complied solutions to common problems relating to workflow notifications.
— Read on community.dynamics.com/gp/b/dynamicsgp/posts/workflow-notification-email-troubleshooting
Workflow Notification Email Troubleshooting – Microsoft Dynamics GP Community
Install, Upgrade & AdminMicrosoft & the GP Ecosystem
Archived post from 2019. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.