I have the same desire.
The best way I have found is to go to your settings (click your avatar in the top right and choose settings).
In here you can choose at the very top between Options, Email Digests, and Notifications. On the Options tab you can scroll down and click no (turning it to a yes) for any forums of any groups you belong to.
On the email digest tab you can set the frequency on which you'd like those to be sent for any groups you belong to.
The Notifications tab allows you to set how you are notified and when. From what I've found there is no way to reply in email other than clicking on the link in the email that is sent and responding there (which is what i did to respond to your message).
Remember though that the above tabs only consider groups you already belong to. If you want to subscribe to a forum or group you haven't joined you have to go to that forum or group page to do so.
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