The next update to the account control center will remove free Secondary DNS and add the “Basic” paid service level. Existing Secondary DNS zones won’t be disabled at this time, but no changes will be allowed to them unless you have a paid account. Back in March we posted an announcement that we’re going to start discontinuing free services and this is the first step.
You can find that announcement here: http://www.rollernet.us/wordpress/2016/03/changes-are-coming-maildns-prices-free-accounts-and-spam-filtering/
A link to this announcement has also been shown in the account control center for the past 8 months. Although we’re sure to see some complaints no matter how much lead time we give, eventually we have to move forward. That time has come for Secondary DNS.
UPDATE: Effective June 5, 2017, Secondary DNS has become paid-only.