When comparing Mandrill vs Postmark, the Slant community recommends Postmark for most people. In the question“What are the best transactional email services?” Postmark is ranked 2nd while Mandrill is ranked 3rd. The most important reason people chose Postmark is:
Postmark is designed to send transactional messages. It specifically warns users not to use it to send weekly newsletter -- this way the emails sent by Postmark are less likely to be marked as spam, increasing the likelihood of your messages reaching the users.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Both SMTP and HTTP APIs are easy to setup
Simply set your SMTP server to smtp://user:API_KEY@smtp.mandrillapp.com:465
and you're good to go. There are libraries for using the API in a variety of languages, including Node.js.

Pro Clean UI
The UI is very clean and good-looking overall. While the Analytics may seem basic and not very detailed, the dashboard interface is very clean which also helps with the set up.
The left side of the page has the navigation to different parts of the dashboard. The main view displays all the most important data and analytics that you may need (emails sent, deliverability, reputation etc...). While the navigation itself is pretty straightforward and self-explanatory.
Pro Has sandbox mode for testing
Instead of sending real e-mails out, can use test keys which allows to test without actually sending out real e-mails. These test keys can also safely be shared inside the team since they cannot be used to send real e-mails on your behalf.

Pro Clear API documentation
The API documentation is very clear and makes it pretty easy to set up you application with Mandrill's API quickly and painlessly.
Pro Great mobile apps
Mandrill has companion apps for iOS and Android that allow monitoring the service using a smartphone.
Pro Highly customizable templates
Mandrill has made templates a core part of the product as they simplify the payload when compared to sending the full HTML each time and allow non-developers to design the template.
Mandrill uses the flexible content formatting of MailChimp templates. This paired with the API access to template management, gives the user a great deal of flexibility.
Pro Webhook Reporting
Mandrill provides and Event API also or "webhooks" through which it registers events when they occur. So when an activity occurs, the application server is called along with the activity that occurred. Events include opens, clicks, bounces, deliveries and spam complaints.
Pro User friendly rules engine
Mandrill allows setting up rules in a non-technical way.
Pro Great MailChimp integration
Mandrill is made by the same company that made MailChimp. As such, both products work very well with each other.

Pro Outgoing messages log
You can see the contents of all messages that Mandrill has sent out, for free for the last 24 hours, or for some cents per thousand messages if you need a longer retention period.
Pro Messages are unlikely to be marked as spam
Postmark is designed to send transactional messages. It specifically warns users not to use it to send weekly newsletter -- this way the emails sent by Postmark are less likely to be marked as spam, increasing the likelihood of your messages reaching the users.
Pro DMARC
Supports domain-based message authentication which helps to prevent spammers from using your domain to send email without your permission.
Pro Detailed analytics
Postmark offers very detailed analytics for your emails. For starters, it stores 45 days worth of email logs where you can search to find out which emails were sent, received, who received them, who read them and who marked them as spam.
There are also recipient profiles which keep track of everyone who gets your emails, complete with a list of messages that they have opened, deleted or marked as spam.
Pro Messages are Saved for 45 Days
All outgoing messages are saved for 45 days so you can see what you're sending to who. This is important if you are sending any dynamic content in your emails.
Pro Multiple server locations
Postmark has 5 server locations around the world (3 in the US, 1 in Amsterdam and 1 in SIngapore). This way emails will be sent fast regardless of where your users are.
Cons
Con Has a bad history
When they decided not to support a free tier anymore they've set up a withdrawal period of only 2 months for existing customer before it stopped working (even for people who bought credits to send more than that free tier). Meaning people using it had a month to switch to another solution or start paying $20/month or more.

Con No free plan
There used to be a free plan which offered 12,000 free emails per month. Unfortunately it has now been discounted and there's no other free plan anymore. If you want to use Mandrill, you have to pay $20/month or more.

Con The documentation is split between two sites
The API is documented at https://mandrillapp.com/docs/, while howtos are in the Knowledge Base. Searching on the former for "Handlebars" doesn't find anything useful about using Handlebars.

Con Template rendering doesn't support Handlebars
The templates.render
call doesn't support Handlebars. This makes complex integrations more difficult. You can, however, use handlebars with messages.send
and messages.sendTemplate
.
Con Expensive
Postmark are one of the most expensive (if not the most expensive) services in the market, at $1.5 per 1000 emails.
