> when I was reviewing the code - I didn't see the according methods implemented

\yii\email\VendorMessage allows any public method or field of the “slave” message object be invoked directly from its instance as well as providing virtual properties based on setters/getters.
This means if Swift_Message has a method ‘attachFile’, you can invoke it from the instance of ‘\yii\email\swift\Message’:
<pre><code>$message = new \yii\email\swift\Message();
$message->attachFile(…);</code></pre>
If Swift_Message has method ‘setReplyTo’  - ‘\yii\email\swift\Message’ has a virtual property ‘replyTo’:
<pre><code>$message = new \yii\email\swift\Message();
$message->replyTo = ‘no-reply@mydomain.com’;</code></pre>

I will add all necessary decoration at doc comments for ‘\yii\email\swift\Message’ eventually, so IDE autocomplete and error tracking will work fine.
Right now I need an approval adjustments proposals (or refuse) of this concept, before I will throw additional efforts on it.