All incoming mail is not received on a computer that has Microsoft Forefront Threat Management Gateway (TMG) 2010 Service Pack 1 (SP1) installed.
This issue occurs because the listener of SMTP Publishing stops randomly.
To resolve this issue, install the software update that is described in the following Microsoft Knowledge Base (KB) article:
2288910 Software Update 1 for Microsoft Forefront Threat Management Gateway (TMG) 2010 Service Pack 1
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates