SendGrid vs sendwithus

We chose Postmark as our transactional email service for several reasons:

• Laser-focus (at the time) on transactional email - their success/speed/reliability with delivering transactional email is amazing. Note, they have now branched out and offer marketing/broadcast email services too.

• Developer-friendly - Awesome docs and resources. Their Rail gem integrates directly with ActionMailer so nearly all of our code worked without changes.

• Servers - You can set up "Servers" for different mail streams/workflows to keep things separate and easy to review.

• Bootstrapped - Wildbit (who makes Postmark) is bootstrapped just like the Friendliest.app and they offer a service credit to other bootstrapped startups.

Of course we chose Coresender to send our own transactional emails :) So I thought I'll let you know how we use it.

• We set up separate sending accounts for all company needs, eg. transactional emails, monitoring alerts, time to inbox. We even configured our office printers to send emails through Coresender.

• We have a real-time and extremely usable view into what emails go through each account, so each time anybody reports an email not arriving we're able to assist them in a few seconds

• We utilize our message timeline feature, so we can learn eg. if people are clicking on password reset links

• We always know how many of our onboarding emails are being opened which helps us improve them

• Finally, we have full controll over our suppressions lists, so we can add (and remove!) from them whenever necessary.

To sum up, at Coresender we're eating our own dogfood and it helps us stay connected to the product and understand our customers better.

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?