How can one ensure the delivery of HTML emails via SMTP?

The correct choice centers around properly formatting messages with the correct MIME type settings as this is essential for the delivery of HTML emails via SMTP. MIME, which stands for Multipurpose Internet Mail Extensions, allows email clients to display different types of content rather than just plain text. When sending an HTML email, it is crucial to specify that the content type is "text/html" in the email's headers. This informs the recipient's email client that the email contains HTML content that should be rendered accordingly.

Using plain text formatting would mean sending the email as simple text, stripping away all HTML formatting, which would defeat the purpose of sending an HTML email. Attaching a PDF document does not influence the delivery of HTML content itself and does not pertain to the email formatting necessary for HTML presentation. Enabling binary transfer mode is not relevant in this context for HTML email delivery; it deals more with how data is transmitted rather than how it is formatted and presented to the recipient. Proper MIME type settings are crucial for ensuring your HTML content is displayed correctly in email clients.