Do we have Email feed on certain tags (such as one is interested in tag "algebraic geometry") here?
Phys.SE. has, it is quite nice. I cannot find it here. The "feeds" I found here is connected to Google Reader, or, to Feeder.
Yes, you can get regular emails with all the new questions from specific tag(s).
Go to math.stackexchange.com. Click on "TAGS". Search for the tag you want. Hover over it, and wait for a box to pop up. In the top-right corner of the box, next to rss, click "subscribe". An email will be sent to your email to confirm your filter subscription.
Be aware that this sends you all questions for a tag, not just the unanswered ones, so you may have to search through them a bit if you want to write an answer.