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.