When comparing Mandrill vs Pepipost, the Slant community recommends Mandrill for most people. In the question“What are the best transactional email services?” Mandrill is ranked 3rd while Pepipost is ranked 9th. The most important reason people chose Mandrill is:
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.
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 Pay only for emails that are not opened
While other services make you pay for each email sent, Pepipost's philosophy is that email conversations should be free regardless, this way the pricing model is built around paying for emails that are not open. So it's up to you to make quality emails and make as much users as possible to open them in order to pay less.
Pro Simple APIs to send emails
Simple HTTP APIs, which are easy to understand and integrate. Complete documentation with sample code snippets for different programming languages. Refer the documentation for more details.
Pro Deliverability and strategic consulting
Inline with the philosophy, Pepipost offers deliverability and strategic consulting to clients to help them make most of their marketing investment.
Pro Awesome support
They are available 24x7, and are very happy to help you.
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 Bad support
They don't give attention to small businesses.
Con Slow to approve an account
It seems an option for Mandrill, but "Pepipost" is slow with support as you may have to wait a while for an account to be approved.
Con Unpredictable price
Because of Pepipost's billing plans, it's very hard to predict costs. You never know how many users will open your emails any given time and if a lot of them end up not opening them (which most of the time is true) the price can get really high.
