A post-delivery alert is a security warning shown after a message has already reached the recipient. Instead of blocking content before it lands, the system lets the message through and then marks it as risky, adds a banner, or sends an admin notification. This approach is useful when an attacker hides a lure inside content that is hard to inspect at delivery time, such as a QR code, shortened link, or image-based phishing message.
In cyber security, post-delivery alerts matter because they can interrupt a user before they click, open, or reply, even if the message initially bypassed filters. They also give defenders useful context for triage, such as why the message was flagged and whether similar messages appeared elsewhere. The main limitation is timing: a user may still see and interact with the content before the warning appears, so post-delivery alerts work best as a second layer alongside spam filtering, URL analysis, sandboxing, and user reporting controls.