To remove the SiteMail Sync utility, you have two options.
Open the SiteMail Sync Setup.msi file you had originally downloaded. This time, it will prompt you as follows:
Alternatively, if you are unable to locate the file, then return to the Tools>Options menu in Outlook and select the Other tab. In the Other tab, click Advanced Options, then Add-in Manager. Now unselect the checkbox next to SiteMail Sync and click OK. Restart Outlook.