Had this problem on my production system.
On running the installer, exactly as I have done on many occasions before, I received an error stating that I did not have sufficient permissions to access the \Program Files (x86)Smartertools\Smartermail\MRS\interface\app directory. On cancelling the update, the installer stated that it had rolled back the changes and that system had not changed - not true, many of the the web interface directories were missing, and as a result the site was down. ActiveSync/POP/IMAP/SMTP also stopped working.
I am logged in as the administrator and I got this error even if I reran the installer with "Run as Administrator". The only difference I have on my system is that the main administrator account has been renamed by using a local policy.
The only way I could get the system back up was to terminate the IIS processes, stop the application pool and stop the website in IIS, then rename the MRS directory to MRS.old and rerun the installer. Then everything worked as expected.
Any reason why this update caused so much grief?