The Application Event Log has Event ID 15002 from MSExchangeTransport saying “The resource pressure is constant at High. Statistics… “. The event goes on to tell you that inbound mail submission has stopped, and it’s due to disk space being low on the volume where the queue database is located.
Either empty more space or.
Basically, you have to adjust the Configuration file attached to the transport hub to ensure that it either doesn’t monitor system resources, or that it has a higher tolerance for when the physical server resources drops below a specified limit. Changing the config file found at C:\Program Files\Microsoft\Exchange Server\Bin\EdgeTransport.exe.config will help with this and allow the server to resume normal functionality.