According to twitter and mailop Network Solutions is having issues with inbound mail, with both TCP level disconnections and 451 deferrals.
@annaciamp @netsolcares We're seeing email from our servers to netsol queuing up, consistently since 9:51 am (U.S. Central)
— Frank Bulk (@frankbulk) October 14, 2015
Down Detector and other reports suggest it’s been an issue since about 8am Pacific Time.
(Thanks for mentioning it, Frank.)