How does the import work?

The data import runs by pulling customer, product, and order data from Magento over the MageMail API in the MageMail connector extension.

it runs every 5 to 10 minutes, and imports new and updated records from:

Newsletter Subscribers
Customers
Products
Orders
Order States
Order Items
Categories
Coupons
MageMail v1 unsubscribes

The way that it looks for updates made to existing records is by looking at the updated_at field on the respective record (customer, order, product, etc.).