I've rolled my own email validation function before but have a project that uses a Pre-Google-Cloud-SDK api:
from google.appengine.api.mail import IsEmailValid
This takes a string and returns True/False if it does/doesn't follow the format for an email address. it doesn't test to see if the email address is live - it only parses the string.
Does this functionality exist in Google's Cloud SDK api?
I suspect not as bulk mail support was dropped from App Engine and, with it, this support function.