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When I upgraded to SM v15.1.6005 earlier this week, I noticed that some part of the upgrade process is removing the custom error page in IIS that redirects from http to https to force everyone to use a secure connection. I'm not sure if this is part of the SM uninstall, or the SM install, but I had to manually recreate this custom error page. The same thing happened on my test server when I was doing initial testing.
I'm referring to Internet Information Services (IIS) Manager > Sites > SmarterMail > IIS > Error Pages, and specifically to a custom 403.4 redirect on that page.
It's not difficult to fix it, but it's annoying that I have to. Is there some reason why the SM uninstall or SM install is deleting this redirect. Note that I'm not sure where in the process it's happening, but nothing else in the instructions touches IIS, so those seem like the two most likely sources of the issue.