The Qbot trojan is again stealing reply-chain emails that can be used to camouflage malware-riddled emails as parts of previous conversations in future malicious spam campaigns. Qbot has been a malware of choice for the notorious Emotet botnet and has seen a surge of new infections. The trojan continues to employ a tactic previously used by the Gozi ISFB banking trojan, the URSNIF information-stealing trojan and the EmOTet trojan.
Source: https://www.bleepingcomputer.com/news/security/qbot-steals-your-email-threads-again-to-infect-other-victims/