The following are exempt from greylisting:
Anyone on your trusted senders list is not subject to greylisting.
Whitelisted IPs for SMTP are not subject to greylisting
Any email user who authenticates is not subject to greylisting
SMTP Auth Bypass list is not subject to greylisting
Anyone who has already sent you an email is not subject to greylisting
Any IP in the greylistBypass.xml file is not subject to greylisting
For more info on greylisting visit http://www.greylisting.org/