Subscribing to tags in WP and getting email digests.

We've been looking for a tag email subscription plugin that does the following:

- Allows users to subscribe to certain tags by entering their email.
- Users can subscribe to one or more tags.
- When a new post of the desired tag is published, the subscribed users get an email.
- Moreover users have the choice to opt for a digest. And if they want a daily digest, then they get one email with all the posts from their subscribed tags that were published in the last 24 hours.

We couldn't find such a plugin, and it's ok if none exists that fulfills all the requirements above. Any pointers for any partial solutions or ideas for implementation could help us a great deal.