In 9 out of 10 cases, this error means that your Microsoft Exchange Server has less than 4 gigabytes of free disk space.  By default, Exchange will defer mail if the server is low on disk space, memory or CPU resources--but disk space is by far the most common cause of this error.

After you've freed up some disk space your mail should begin to deliver again.  Once you've received a few messages and you know the problem is fixed, login to the SpamHero control panel and go to the Tools tab and then use the Flush Mail Queue tool to have all of your deferred mail re-queued for immediate delivery.