SMTP smuggling is a protocol framing flaw in which different mail servers disagree about where one SMTP message or command sequence ends and the next begins. Because SMTP is often parsed by multiple layers, a small difference in handling line breaks, dot-stuffing, or command boundaries can make one system accept traffic that another system interprets differently.
This matters because attackers can use the disagreement to inject extra content, split a message unexpectedly, or bypass filters that rely on consistent parsing. In practice, the issue often appears at trust boundaries such as mail gateways, relays, and proxy front ends. Defenders reduce risk by keeping SMTP implementations patched, normalizing input at a single trusted layer, and testing how relays, proxies, and back-end servers interpret edge-case message boundaries. Careful parser consistency is critical in mail security, where a tiny framing mismatch can change what gets delivered.