First releases of mailer package and its SwiftMailer adapter were released.
The mailer package provides the content composition functionality, and a basic interface for sending emails.
Actual mail sending is provided by separate interchangeable packages.
The following code can be used to send an email:
/**
* @var \Yiisoft\Mailer\MailerInterface $mailer
*/

$message = $mailer->compose()
->withFrom('from@domain.com')
->withTo('to@domain.com')
->withSubject('Message subject')
->withTextBody('Plain text content')
->withHtmlBody('HTML content')
;
$mailer->send($message);

Out of the box the package profiles a file mailer that, instead of actually sending an email,
writes its contents into a file. There is also SwiftMailer-based official driver available as a separate package that actually can send emails.