Exchange 2013
From Roaring Penguin
Exchange 2013 stopped receiving mail periodically (every 3 - 8 hours) with the telnet session reporting: 421 4.3.2.-service-not-available
This is a known bug/behaviour in Exchange 2013. A Google search revealed:
http://llamait.wordpress.com/2013/06/13/exchange-2013-421-4-3-2-service-not-available/ http://social.technet.microsoft.com/Forums/exchange/en-US/606e823b-74ee-486c-b074-9d88c69904f5/421-432-service-not-available http://social.technet.microsoft.com/Forums/exchange/en-US/cd03793b-9e99-474e-8f25-11275652882f/exchange-2013-stops-receiving-external-email-after-about-8-hours
A workaround that may potentially be useful can be found here:
https://helpdesk.spamtitan.com/support/solutions/articles/4000003763-dynamic-recipient-verification-using-exchange-2013-and-2016