WASI has been in Preview1 for a couple of years now, indeed. However, Preview2 is well underway and the initial implementation is scheduled to land in wasmtime this June.
An email sandboxing and testing application called MailSpons: https://mailspons.com/
Primarily, it allows developers to set up fake inboxes for their local development environments.
The only thing you have to do is change the SMTP settings of your local environment and then all mails will be caught by MailSpons.
The next step is to automate end-to-end testing of applications by reading the sent emails via the API. I am currently writing the documentation for that.
Please don't be put off by the homepage. I, a developer, "designed" it ;)