Yii team released SwiftMailer extension version 2.0.7. Changes are the following:

• Ability to setup several message headers in batch, including configuration at yii\swiftmailer\Mailer::$messageConfig.
• Better logging support, by choosing logging level (info, trace, warning) according to swiftmailer log format.
• Fixed bug: yii\swiftmailer\Message did not clone the underlying $swiftMessage instance during its own cloning.