SMTP smuggling feels similar to HTTP smuggling, but differs in impact.

SMTP smuggling sends a legitimate email to an unsuspecting receiver. It ends up in someone's inbox, at another provider.

HTTP request smuggling is not always visible to other users, and can (depending on the web app) be limited to one backend cluster and the attacker's own requests, i.e. to steal/manipulate private data.

There are multiple HTTP smuggling vulns by now.

A recent one from DEFCON 27:

https://youtu.be/w-eJM2Pc0KI