Greylisting is an anti-spam technique. When a server you've never received from sends a message, the system temporarily replies try again later. Legitimate mail servers automatically retry within a few minutes and the message is then accepted; many spam systems don't retry, so spam is blocked.
The side effect is that the very first email from a new sender can be delayed by a few minutes up to about an hour. After that first message, the sender is trusted and future mail arrives normally.
If a particular important sender is delayed every time, you can disable greylisting in cPanel under Configure Greylisting (if available) or add their domain to the bypass list. If you can't find that option or delays are excessive, please open a ticket and we'll adjust it for you.
