We used Couchbase (a NoSQL like MongoDB) for email batch sending (think invitations to a party) and it worked very well. We could store each email sent as a separate, denormalized document so the sender could see EXACTLY how their contact data was replaced in each individual instance, the "View as a Web Page" functionality was trivial (instead of recalculating everything from the normalized forms- which can be blown up by contact data changes, just load the document that you sent out), and it's lovely TTL feature meant we could handle the configurable retention policies trivially as well.

It wasn't so good at doing reports (how many customers viewed the email yet, responded, etc.). One thing we talked about doing was just storing a sqlite or h2 database as a document for reporting purposes (if we had been more single threaded that could have worked nicely). We ended up using a separate Sql DB for that.

There are cases where denormalized data is the "right" way to view stuff, and cases where the data really is easier if its normalized, and that is a good reason to push your DB selection one way or another.