Kace Agents Not Checking in after SMTP change
A change to the SMTP settings would not affect the Agent communications.
You may want to verify all the required services are running on your appliance.
Go to Settings -> Support -> Diagnostic Utilities. Select services in the dropdown menu.
They all should have a "ok" next to the service name.
The next step would be to collect the Agent logs from one or more devices for review.
The KAT (Kace Agent Toolkit) can help you with this process. https://support.quest.com/kace-systems-management-appliance/kb/263376/using-the-kace-agent-toolkit-kat-