mail\BaseMailer $messageConfig

The configuration that should be applied to any newly created email message instance by createMessage() or compose(). Any valid property defined by yii\mail\MessageInterface can be configured, such as from, to, subject, textBody, htmlBody, etc.

For example:

[
    'charset' => 'UTF-8',
    'from' => 'noreply@mydomain.com',
    'bcc' => 'developer@mydomain.com',
]