Mails from Lotus Notes/Domino were backing up on the Domino server and failing with "Transient error received from remote server."
Mails were queuing in the mail.box on the Domino server.
This is caused by a System resource monitoring feature in Exchange called Back Pressure.
The following is an extract from Microsoft's Technet document on the subject:
Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on Microsoft Exchange Server 2010 Hub Transport and Edge Transport servers. Exchange transport can detect when vital resources, such as available hard disk space and memory, are under pressure, and take action in an attempt to prevent service unavailability.
This is not a Quest /CMN issue and needs Microsoft's assistance to resolve.
