451 greylisted message from customer

I had a potential client receive this message below. How do I disable greylisting until I feel comfortable it is not causing more harm than good? I do not see a security button when I am logged in which is what the help file says to access. I see the settings tree on the left but no choice to disable greylisting.
Thanks,
Gil

451 Greylisted, please try again in 710 seconds


Failed to deliver to domain nssdd.com after 3 tries.
Last error was:
No recipients were successfully delivered to.