Getting more emails?

Hey everyone, we've been using Aiwyn for a while but internally we are still challenged with getting more billing contact emails in place in our system. 

For those who have increased their emails before or since going live with Aiwyn, do you have any tips or tricks to share? 

We're requiring emails now for new clients, which is working great, but I'm still not sure of the best way to get more emails for those existing clients. Any suggestions?