Our MSI's and EXE's used to be functionally different from eachother and would actually cause issues if one was installed over the other. However, starting with the new 17, the MSI and EXE are, to the system, the same so you can install one or the other. We found out last night that the MSI installs waayyyy faster than the EXE does when doing an upgrade on our SmarterTools box.

However, this is where they differ. The EXE handles licensing the product (if you aren't already licensed), installs the pre-requisites, Adds the website to IIS, Bounces the SM app pool.
But, if you've already licensed your product, added the site to IIS, and you have the pre-reqs. You can totally use the MSI now.

THIS IS ONLY 17/100 and UP! Do not do this with an older version of SmarterMail, otherwise your gonna have to repair your registry and hack the duplicate install out of windows.