I found it after finishing installation (technically, the installation directed me to turn it on). Turned it on.
The test email I send does, indeed, now have an HTTPS address, but merely navigating to the site still show the site icons as HTTP, and thus the mixed contents warning still shows.
Now, this might be because I did not update the icons, but I don’t have icons to put there just yet. Even so, I don’t understand why it should be like that.
Also, I tried turning it off and on again (ha!), but that didn’t solve the problem either.