People who got their hands really wet with exchange 2007/2010 has a good amount idea about Exchange back pressure. Basically it is a kind of situation where Microsoft Exchange stops receiving emails. This is a new condition introduced in Exchange 2007 and improved in Exchange 2010 for better handling of emails.
Exchange server feels this back pressure when it is out of resources. This resource outage can be of two types 1) Disk out of space 2) High Memory utilization
See this video to understand it better.
[Video Source : www.insidetheregistry.com]