Aric Fedida

I recommend PostgreSQL as it’s the most powerful out of the 3 databases you mentioned. It supports JSON objects so you can mimic the MongoDB functionality, but I would also argue that SQL is actually quite powerful and in many cases significantly easier to work with than with NoSQL databases.

Stay away from foreign keys, keep it fast and simple. Define your data structures well in advance. Try to model your data structures based on your system’s vision; based on where it’s going and not based solely on what you currently need it to do. This will help you avoid drastic changes to your database after your system is launched. Populate the database with fake data and run tests. PostgreSQL allows you to create Views from multiple tables. Try to create those views and make sure you can easily create useful views from multiple tables. Run an Explain on those view queries to make sure you created your indexes correctly. Make sure it’s fast!

At this stage, all of the tools you mentioned do email delivery pretty well. They all support email templates as well. Here are some considerations:

1. Twilio owns SendGrid. If you're an existing Twilio customer, in my opinion that's a good reason to use SendGrid over the other solutions. The APIs are solid, and Twilio has excellent developer tools that allow you to create interesting automations (which is important for scaling).
2. Mandrill was created by MailChimp, who have massive experience with email delivery and specifically with emailing beautiful email templates.
3. Mailgun is a tool on its own. Like the other two, it supports mail templates and is built to be controlled almost exclusively via APIs.

SendGrid and Mandrill have pretty nice WYSIWIG template editors as part of their platform. Not so sure about Mailgun.

So for me the considerations would be: 1. How easy is it for you to integrate with their API? How complete is their API in terms of your own specific needs? 2. Prices: Which one works best for my budget? 3. Am I OK with editing the templates elsewhere (or even by hand), and then pasting the code into Mailgun? Or do I want the comfort of Mandrill or Sendgrid with their WYSIWYG editors?

Personally I'd go with Twilio, simply because it's such a massive ecosystem they are less likely to go bankrupt, and their APIs are rock solid.