Email deferred due to greylisting - SPF: 'unchecked'

Symptoms

You may see the following in WHM >> Mail Delivery Reports under the Results column of an email, received from a remote mail server:

Deferred due to greylisting.
Host: 'RemoteServerIP'
From: 'remoteemail@remotedomain.tld'
To: 'localemail@localdomain.tld'
SPF: 'unchecked'

Description 

This occurs when greylisting is enabled in WHM >> Greylisting, and the option Bypass Greylisting for Hosts with Valid SPF Records is disabled, or the remote server is not listed in Trusted Hosts.

Workaround

Option 1: If the remote domain has a valid SPF record, enable Bypass Greylisting for Hosts with Valid SPF Records in WHM >> Greylisting. 

Option 2: Add the remote mail server's IP address to the Trusted Hosts list in WHM >> Greylisting >> Trusted Hosts. You can also add the IP address of the remote mail server to the Trusted Hosts list in WHM >> Greylisting >> Reports >> Click the plus '+' icon to the far right and then select the IP address of the remote mail server to add it to the Trusted Hosts list.

Option 3: Disable Greylisting in WHM >> Greylisting.