I did a quick search but only found someone saying it doesn’t work. But I’m not entirely sure what it’s used for in the first place. Can I get a short explanation?
It’s for bounce handling, when email fails to deliver, we want to disable the user’s email and/or notify them via PM that their email is busted. You’ll notice several common email services are represented in site settings, for the same reasons.
In order to know that email failed to deliver, you need to configure the email service to tell Discourse that, because otherwise, how would it possibly know?
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More information available on the dedicated howto topic 
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That’s perfect, thanks guys
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