Craigslist has implemented this perfectly for years but I don't see a lot of other companies doing it. It makes a lot of sense that Craigslist doesn't need to filter / moderate messages between users as it just provides a way for them to communicate through email (mostly) anonymously.

Creating such an email relay seems non-trivial so I'm curious if there are any open source or even paid products that provide the same service. Does anyone know?