Contacts and contact lists in Mixmax are separate from those in Google Contacts. When you first sign up for Mixmax, we import all your contacts from Gmail. If you want to sync your contacts again, here's how to re-sync contacts from Google Contacts.
If you don't see contacts from your company directory or address book, here's how to sync your company address book.
Note that Mixmax has Contact Lists, which make it easy to email a group of people — and get accurate analytics on who opens, clicks, and downloads.