Magento 2.3 requires `zendframework/zend-mail` package (the Mail component from Zend Framework 2) since 2017-07-06

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https://github.com/magento/magento2/commit/e7f62fef

https://github.com/magento/magento2/blob/e7f62fef3/composer.json#L20-L20

Magento < 2.3 does not require zendframework/zend-mail.