How do you use a transactional email service like Mandrill or SendGrid to send emails instead of the builtin server based smtp?
Thats a very interesting question i'd like to find out. On this way it might be possible to adress users by different actions, like when they do not return over a longer time period or when they just entered the forum.
Check the AddOn repository, there is a new MailChimp integration plugin. Try this one out, maybe you can find out more about this and let us know.
You can send through any SMTP including Mandrill's so long as your server allows it. That is not to say they wouldn't shut you down if they didn't like the activity.
If you want to use an api, then you would have to put something in place of library/core/class.email.php
grep is your friend.