There are two concepts here
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When do you get emailed
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When do you get a Discourse notification
They are two different and somewhat unrelated topics.
Note that Discourse won’t email you about a notification if you are browsing the site and clicking / tapping around – because that would be double-notifying you. There are also some notifications that Discourse doesn’t think are worthy of an email notification, ever.
The general philosophy is that
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you will get notified when someone is speaking directly to you.
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you will get an email about that notification if it is significant (e.g. someone is talking directly to you), and you are away from the website – no devices are actively pinging the website while logged in as you.
It is possible to dial this to 11 if you want an email for everything. You might have a look at the email preferences in your user profile to get a sense of where you want that to be for your account. But out of the box, it works as I describe 
.