I recently installed the SMTP service on a new server and it would not start. I would receive an error to check the event log but nothing was even logged. What ended up fixing the problem was the following:
Check this registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SMTPSVC for a dword value called Start. If it’s not there, create a dword key called Start and set the value to 2.
I had the key but the value was set to 4. I changed it to be 2 and restarted the server and all worked as it should. Weird eh???
As always use caution when you edit the registry and backup your system first.