What is Follow-up email?
A later step in a sequence, usually threaded as a reply to the first email, that adds a new reason to answer.
Most replies to cold email come from follow-ups, because the first message arrived at the wrong moment for most people. A follow-up should be two or three lines, in the same thread, at least three days after the previous step, and should add something — a result, an example, a question — rather than "just checking in". Three follow-ups is usually the limit; beyond that replies stop and complaints start.