Allow sending of the transactional email via Mandrill or other Email Service Provider
Two reasons: 1) Outbound emails are sent out into the interwebs and we have no idea if they actually where delivered and/or open. It's important to me to know if my customers get their emails from the UTM. EMail service providers help bridge that gap. I realize that the need is likely counter to current thought, but many of my customers are using cloud based email services and there are more potential issues that could prevent delivery of the UTM emails. 2) Not all of my customers use "business grade" internet links because they really don't need them and often the SMTP ports are blocked by the ISP. Mandrill (and other ESP's) have generous & free access to their transactional email services. Outbound Emails are simply API calls.
The emails that the UTM's are important and using an esp for delivery is one way to ensure they are delivered and/or read.