How to activate email addresses during import
If you check the Activate email addresses option, the imported addresses will be active. If you do not check this option, they will be inactive. 🙂
The most important difference between the two is that mass emails can be only sent to active email addresses.
If the owners of the email addresses to be imported have not subscribed for your newsletters yet or they have not received mass emails from you for months, you have to import the email addresses as inactive.
Later on you can activate them by sending a custom email to them.