Greylisting is a spam-filtering method where the receiving server temporarily refuses an email from a sender it hasn't seen before, with a "try again later" response. A real mail server retries a few minutes later and is let through; much spam software never retries.
What you'll notice
A delay of a few minutes to an hour on the first email to a new recipient, then normal delivery. In the header, the Received times show the gap.
Does it hurt cold email?
Not when you send from proper mailboxes, such as Google Workspace, which retry automatically. It only matters if a tool treats the first temporary refusal as a bounce or doesn't retry. It's one reason a delivery delay on a first email isn't necessarily a deliverability problem.