We have a client that is using AppRiver to scrub email before it is delivered to their on premise Exchange 2010 server. Throughout the day AppRiver will stop delivering email and the message states
"Our last attempt to connect to your domain has failed. Your mail server might be refusing connections from our Delivery Server.
Please ensure that your mail server is accepting inbound SMTP connections"
When that happens within a few minutes the email will start flowing again to the Exchange server and we have made no configuration change.
Any help would be appreciated. I have went through all steps in this document listed.