A few days ago, we changed the IP address of ns2.messagingengine.com from 126.96.36.199 to 188.8.131.52. In general, this shouldn't have affected any users because we tell users to point their domains to our name server records ns1.messagingengine.com and ns2.messagingengine.com.
However we've noticed that a few users have slightly esoteric DNS setups where they've used the explicit IP address of the old ns2.messagingengine.com server. For those users, you need to update your DNS setup. We highly recommend that if possible, you don't ever use the explicit IP address, but use appropriate server names or CNAMEs. This allows us to change the IP addresses of ns1 or ns2 without any user changes being required.