Unfork zetacomponents/mail (also helps Drupal8)
zetacomponents/mail was forked in early 2017 for PHP 7 compatibility. However, the base branch has had updates since which fix the PHP issues, and also add a new features for STARTTLS that might be interesting (and solves issues such as: https://github.com/civicrm/zetacomponents-mail/pull/2)
I encounted this issue while playing around with composer and Drupal8 from scratch:
$ composer create-project drupal-composer/drupal-project:8.x-dev myproject --stability dev --no-interaction $ cd myproject $ composer require civicrm/civicrm-core:5.7.2 --prefer-dist ./composer.json has been updated > DrupalProject\composer\ScriptHandler::checkComposerVersion Loading composer repositories with package information Updating dependencies (including require-dev) Your requirements could not be resolved to an installable set of packages. Problem 1 - Installation request for civicrm/civicrm-core 5.7.2 -> satisfiable by civicrm/civicrm-core[5.7.2]. - civicrm/civicrm-core 5.7.2 requires zetacomponents/mail dev-1.7-civi -> no matching package found. Potential causes: - A typo in the package name - The package is not available in a stable-enough version according to your minimum-stability setting see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details. [...]
This can be fixed by specifying the URL of CiviCRM's fork, but eh, if we can remove it, why not?