title: Symfony Mailer - Migrate addPart from SwiftMailer
body: <p>I am migrating from SwiftMailer to Symfony Mailer in my application.</p>
  <p>How to migrate <code>addPart</code> method of SwiftMailer to Symfony Mailer? In my case the content type is <code>text/plain</code></p>
  <pre><code>-&gt;addPart('Plain text content', 'text/plain');
  </code></pre>
  <p>using</p>
  <ul>
  <li>swiftmailer 6.1</li>
  <li>symfony mailer 4.4</li>
  </ul>