You may have addresses on hold if the number of records in your Master List exceed the number allowed by your monthly subscription. To mail to these addresses you can upgrade your monthly subscription by following these steps.
1.) Login to your account
2.) Click Account | Subscriptions
3.) Click on the blue Actions link to the right of the active subscription
4.) Choose the option Modify Subscription
5.) Choose the appropriate tier and proceed to the payment page
Note: you will be credited for the unused portion of your existing subscription and that credit will be applied to the cost of a new