For transactional emails, notifications, reminders, etc, I want to make it so writers/designers can set up the emails and maintain them, and then dynamically insert fields, that I then replace when actually sending the mail from code.

I think the ability to use a basic layout template across individual email templates would make things a lot easier (think header, footer, standard typography, etc).

What is best for this? Why would you prefer Mailgun, SendGrid, Mandrill or something else?