Well, in order to have threads check to see if mail bounces, it'd have to either:
- Have an email pipe in your aliases config on your server so that when mail is recieved it is processed by threads to suspend or delete an account.
- Act as a pop3 client and download email from an account and process emails accordinly
Both well beyond the scope of threads at this point (as I think there are some major things that are on the docket to be worked on before dealing with bounced emails (as if a user doesn't approve their email working their details will be purged at registration).