Email deferred due to greylisting - SPF: 'unchecked'

Symptoms

You may see the following in "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, and the "Bypass Greylisting for Hosts with Valid SPF Records" option is disabled or the remote server is not listed in "Trusted Hosts."

Workaround

Option 1: Enable the "Bypass Greylisting for Hosts with Valid SPF Records" option if the remote domain has a valid SPF record.

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Email / Greylisting."
3. Select the "Bypass Greylisting for Hosts with Valid SPF Records" option. 
4. Click the "Save" Button.

Option 2: Add the remote mail server's IP address to the "Trusted Hosts" list.

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Email / Greylisting."
3. Click the "Trusted Hosts" tab.
4. Enter the remote mail server's IP address in the "New Trusted Hosts" text box.
5. Optionally, add a comment about the remote mail server's IP address in the "Comment" text box.
6. Click the "Add" button.

Option 3: Disable Greylisting.

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Email / Greylisting."
3. Click the "On/Off" toggle next to "Greylisting is Enabled."

Additional resources

How do I verify my SPF records are working?