Our small network (1 server, ~10 workstations) runs Exchange Server 2010 on Windows 2008 R2 Standard; it has worked fine for years, but recently we cannot send or receive mail, though no error is apparent in Outlook; mail just ceases to arrive or be delivered. (External senders get a failed delivery notification after some hours.)
Disk space and memory look okay (store.exe uses ~90% of RAM but I gather this is normal and it will release RAM as needed); I see nothing of relevance in the Application event logs. Exchange Best Practices Analyzer shows no critical issues.
The Troubleshooting Assistant's Connectivity Test indicates that "the SMTP service and SMTP instance(s) on server MAT1 are started but the port did not respond". Further investigation shows that the Windows service "Microsoft Exchange Transport" is hanging (and due to the hang, cannot be stopped through the UI, only by taskkill). Rebooting the server fixes the issue briefly, but the service always hangs again a few hours later. How can I diagnose this hung service?