Also, we need to find a name for "give me some personal data in return for a minimal value add service" offerings.
I have been using the same regex[1] for years it gets the job done adequately--at least, I've never received any complaints from users or clients.
I've used the regex on some relatively high profile sites--the kind where if someone was unable to signup with a valid email address we would've definitely heard complaints.
"No addresses submitted to the guardpost service are ever stored on any Rackspace servers. Nothing is persisted after the request is complete."
However, the API is using GET.
So unless you turned off logging, you are storing all tested emails in your web server logs which most places gzip up and archive indefinitely. In other words, I'd imagine these logs "persist" after the request is complete.
ADDED: As an end user, you may or may not be considered that these logs will contain:
1. Your email address
2. The place you are making an account
3. The time you created the account
And this information will be stored likely indefinitely (whatever the server log retention policy is). These logs also give mailgun and/or Rackspace a great resource for the membership rolls and adoption rate of any site adopting this service.