MMC has detected an error in a snap-in. It is recommended that you shut down and restart MMC

We found that doing the following worked, and can now edit the settings in the II6 Manager without errors (and without crashing when applying settings) every time:

  1. Stop SMTPSVC service [Display Name: Simple Mail Transfer Protocol (SMTP)]
  2. Stop IISADMIN service [Display name: IIS Admin Service]
  3. Edit "C:\Windows\System32\inetsrv\MetaBase.xml"
  4. Find: <IIsSmtpServer Location ="/LM/SmtpSvc/1"
  5. Add (Settings are alphabetical): RelayIpList=""
  6. Save file
  7. Start IISAdmin Service
  8. Start SMTPSVC service

You can now open the Internet Information Services (IIS) 6.0 Manager (InetMgr6.exe) and work like normal. The setting above relates to the Access Tab > Relay Restrictions section, but doesn't set anything in it (i.e. "all except the below list" with a blank list).

After setting this value you can change it to what you need it to be. (i.e. the value doesn't need to be 'null' for the fix to work, just so long as the variable is present in the file).

The above has been tested on a few new installs of Windows Server 2022, and has worked for us every time. If you have done an upgrade there are some other things to consider (seems like the SMTP service is not installed after upgrade so will require a backup of the settings that can be applied post reinstalling the service. But that is discussed in other topics).

PS: Remember to set the SMTPSVC service to start "automatically". By default its set to "manual".

Also Here: How do I resolve the "Snapin Error" when setting up SMTP on IIS Server? - Server Fault

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