Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and click on General setting.
Step::3 Now go to “Other” tab in general setting page.
Step::4 Tick on the Marketing emails option.
Step::5 Click on Save changes.
That's It !
To know the above detailed procedure of above discussed steps, check out the video tutorial. You may read the blog as well