I know, what a transactional email is (like sending reset password, sending welcome message confirmation upon registering, etc...).
My doubt is the survey mails, are these considered transactional emails?
For example, I have a survey and I need to send it to a bulk of customers, will that be treated as a transactional email? If yes, then can I use Mandrill email service provide for that?
Surveys would fall under a 'request' email, which is still valid as a transactional email if it's done properly. Other types of request emails include asking users for feedback, or to rate things such as products or services. You should be fine to do this.
Here is a great article from Vero that includes some thoughts on 'request' emails.
Finally in answer to the second part of the questions, yes you could use SendGrid or Mandrill, to move this kind of email to your users.
