Its a long pending requirement to enhance the email capability on Q2A. Some of the requirements I'll add below. Please feel free to add more.
1. HTML email support with HTML email templates for common emails like answer received, new user email etc.
2. Currently Q2A emails are send to all users who commented to a post even when 100s of comments have happened. Probably this should be limited to last 10 or last X number of unique users.
3. Option to limit number of emails a day per user as told in the question
4. Option to add a summary email per day/per week
5. Option to opt out of different kinds of emails per user.