The subject problem will occur if the SMTP (Simple Mail Transfer Protocol) transport event sink is processing asynchronous events when you close the IIS service, because the memory that the SMTP transport event sink is using is released when IIS closes.

To work around this issue, use the iisreset.exe command to STOP or RESTART the IIS service.

When I type iisreset /? in a CMD.EXE window, I receive:

IISRESET.EXE (c) Microsoft Corp. 1998-1999                              Usage:                              iisreset \[computername\]                                  /RESTART            Stop and then restart all Internet services.                                  /START              Start all Internet services.                                  /STOP               Stop all Internet services.                                  /REBOOT             Reboot the computer.                                  /REBOOTONERROR      Reboot the computer if an error occurs when starting,                                                      stopping, or restarting Internet services.                                  /NOFORCE            Do not forcefully terminate Internet services if                                                      attempting to stop them gracefully fails.                                  /TIMEOUT:val        Specify the timeout value ( in seconds ) to wait for                                                      a successful stop of Internet services. On expiration                                                      of this timeout the computer can be rebooted if                                                      the /REBOOTONERROR parameter is specified.                                                      The default value is 20s for restart, 60s for stop,                                                      and 0s for reboot.                                  /STATUS             Display the status of all Internet services.                                  /ENABLE             Enable restarting of Internet Services                                                      on the local system.                                  /DISABLE            Disable restarting of Internet Services                                                      on the local system.