This extension is a wrapper for sending your Opencart emails with Mandrill API.

Easy to install from Opencart admin:

1. Go to: Extensions -> Extension Installer: select file: mandrill-*.ocmod.xml
2. Go to: Extensions -> Modifications: click Refresh Modifications. 
3. Go to: System -> Settings:
   1. Mail Engine: Mandrill (SMTP).  
   2. SMPT Password: Mandrill API Key.  (Copy from your SMTP credentials on the SMTP & API Info page in your Mandrill account)

NOTE: You need to Verify Your Domain from your Mandrill account.

This extension is built as a simple wrapper with minimal changes but Mandrill offers some awesome advanced features that can easily be added with a little coding knowledge see here: https://mandrillapp.com/api/docs/ 
Check the main wrapper file: /system/library/mail/smtp_mandrill.php