The Email Delivery options control how you wish to receive group emails. All Messages will send each email individually, as soon as it's posted. Daily Digest will group the days messages into a single message. As soon as 25 messages are sent to the group, or every evening, whichever comes first, a digest message will be sent to you. You can also opt to not receive any email.
Clicking Advanced Preferences will expose additional options. These options filter the types of emails that you will receive.
Muting and Following
In addition to the above subscription options, you have the option to mute individual discussion threads and hashtags. In the footer of each email is a link to mute the thread. And from the website, you can mute hashtags.