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.