We don't see and remnants of usage on our server in the Windows\Temp folder - everything temporary seems to reside in the SmarterMail\Service\App_Data - but we have no MAPI usage -
Is this on the current build?
Is this an upgraded system that possibly has leftovers laying around? We found as SM has changed over the last couple years that there were multiple leftover folders from old versions after upgrading that should no longer be necessary.
Is it a solid fact that the files in Windows\Temp are actually from SmarterMail, as this folder is the literal toilet for Windows leftovers by design - more than one app craps there (so to speak)...
That said, you can set the an environment variable in your Windows settings (Control Panel -> System -> Advanced -> Environment) or (Settings -> System -> About -> Advanced System Settings -> Environment Variables) and at the system level change it to something else.
A solution would be to setup a routine that just deletes old temp files outside of a 30 day window on a daily recurrence and called it a good job --
MailEnable survivor / convert --