Some mail services (for example, Google Apps™) allow customers who own their own domains to relay email through different mail servers. If properly-configured SPF records do not exist for the mail service for these customers’ domains, Greylisting may identify them as spam. To prevent this, add these IP addresses and ranges to the Trusted Hosts list in WHM’s Greylisting interface (WHM >> Home >> Email >> Greylisting).
The following are the common mail services and their system IP addresses and ranges. Click the mail service to view its IP addresses and ranges: