Hey Lenny,
Enabling greylisting is done at a system level; however, you can override the setting for domains and users. Log in as the System Admin, click on the Security icon, then
Greylisting. Here is more info on what it does and what each of the settings are for:
Greylisting.
Once enabled at the system level, you can disable greylisting for all domains but the one you would like. To do this, you simply edit the domain and click on the Technical tab > Disable greylisting. In the system security settings, you can also allow users on that domain to override greylisting, if desired.
I hope this helps!
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com