Because of unbounded requirements, installing drupal/mandrill 7.x with composer installs Drupal 8 core

In drupal packagist, the requirements for drupal/mandrill are:

• drupal/drupal: 7.*
• drupal/mailsystem: >=7.2.0
• drupal/libraries: >=7.2.x-dev

If the project does not specify versions ~7.x for drupal/mailsystem, composer will install drupal/mailsystem version 8 or higher, which requires Drupal 8 core, which causes composer to download all of Drupal 8 core's dependencies as well.

Workaround: add drupal/mailsystem to your root-level composer.json along drupal/mandrill.