When viewing statistics for a sent email campaign, you may see the term "Retry Recipients".
'Retry recipients' are recipients the system has already attempted to send to and the receiving server has returned some kind of message saying, essentially, "I can't accept this right now, try again later."
This message could be triggered because their servers are too busy, they have some kind of greylisting system where they don't take the email on the first attempt, or it could be that something is just down on that recipient's end.
WILL RETRY RECIPIENTS BE ATTEMPTED ANOTHER TIME?
The number of retries can vary, as there are some rules that are specific to different systems that govern how often retries happen. Generally, the system retries until the email either goes through, gets firmly rejected, or the campaign expires (typically 4.5 days).