CHUNKING is an SMTP extension that lets a client send an email message body in separate blocks instead of one uninterrupted stream. The server receives each block with explicit size information, which makes large transfers easier to manage and reduces some buffering pressure.
In cyber security, CHUNKING matters because it changes message framing and server state handling. Mail software must track how many bytes belong to the current chunk, whether the message is complete, and how that data interacts with other layers such as TLS. Bugs in this logic can lead to desynchronization, crashes, or memory corruption. Attackers may probe CHUNKING support to reach parser edge cases, while defenders test it to find fragile transitions and harden mail servers against malformed or unexpected input.