Navigate to Security Settings > Email > Filter Policies.
While on the Inbound tab, click New Filter.
Choose a name for the filter (i.e. Safe Senders) and leave the Direction set to Inbound.
Click Continue.
In the Filter Logic section, choose the proper Scope for the filter:
If you want the filter to be applied to everyone, leave scope = <Company>.
If you want to target a specific individual or group of individuals, change the scope.
In the If section:
First drop-down box: Select Sender Address from the dropdown list of options message elements.
Next drop-down box: Select IS.
Final box: Enter the full email address of the sender (e.g. external-voicemail-system@mydomain.com).
In the Do section: select <Allow> from the dropdown list of actions.
On the right, in the Filter Details box, in the Description box, give a brief description of the filter's purpose.
Click Save.
Troubleshooting tips:
Vendor documentation can be found here.