We are seeing a new type of attack popping up more and more. Bad guys send a phishing attack and steal the credentials of your employee. But they stay under the radar and lurk for a while to understand the email traffic and the people the compromised account regularly talks to.
Refer Here for Original Post and Source https://blog.knowbe4.com/whats-the-best-name-threadjacking-or-man-in-the-inbox-attacks