BTW, also got an installation error on one of the three servers I installed on:
"Unable to create \Smartermail\MRS\Interface\Output"
Sure enough, that folder already existed even after removing the previous version of Smartermail and I did not have privileges to access after the uinstall of the previous version as they were now set to SYSTEM. I couldn't even take ownership of the folder as the Domain Admin. Double-checked to make sure that both the Site & AppPool were stopped in IIS. Rebooting the system cleared the folder and I was able to resume installation again. IIS must have been holding on to that folder for some reason (it may be important to note that we don't do an "IISRESET /STOP" any longer as it can cause corruption in IIS 7.0 & higher, as per Microsoft's warning to no longer use this cmd. We only use "appcmd stop apppool /apppool.name: Smartermail" and if that isn't enough a "net stop WAS /Y"). Still, despite not seeing that installation error before it was pretty easy to realize that a Reboot was probably all that was needed, although non-I.T. personnel might be stumped by the installation error.