if your building a social app and want users to invite their email contacts to join, you could try to build integrations with all the likely email services or you could just use the ShuttleCloud API, which the company says can currently import address books from 242 email providers.
Also provides robust data analysis on email metadata to surface insights about relationships, such as recency and frequency of contact. For ex, instead of just presenting you with a long list of contacts, ShuttleCloud might suggest the friends who are most likely to say yes to an invite.
ShuttleCloud
Petit Hacks