Twitter has hired the team behind RestEngine, a 2.5-year-old company that focuses on e-mail marketing.

RestEngine describes itself as a "social marketing automation solution" aimed at app developers and publishers. The company serves up personalized marketing e-mails. Some of its customers include RockYou, UStream, and CrowdStar.

TechCrunch first reported news of the deal, which the outlet says was completed in late April. RestEngine alerted users about the move in a note on its Web site today:

We're very excited to announce that the RestEngine team is joining the Twitter flock! Just over two and half years ago we founded RestEngine to help social app publishers send targeted one-to-one emails based on a subscriber's social graph. It's been an incredible journey where we had the opportunity to work with some of the leading social app publishers. With our customers, we've iterated on our social marketing automation platform while defining a new set of best practices for this brand new world of outbound social marketing. We're thrilled to now focus our email skills and marketing automation know-how on a much larger scale at Twitter.

Per TechCrunch, RestEngine co-founder Joe Waltman is not making the move to Twitter, and the existing RestEngine site is being shuttered.

Twitter's been on a buying and hiring spree in the past few months. Last month the company bought social-media analytics startup Hotspots.io. Before that, the microblogging service bought Posterous, Summify, and TweetDeck.