apply_filters( 'wp_mail_content_type', string $content_type )

More Information

• The default content type for email sent through the wp_mail() function is ‘text/plain‘ which does not allow using HTML. However, you can use the wp_mail_content_type filter to change the default content type of the email.
• In general, content type is going to be ‘text/plain‘ as the default, or ‘text/html‘ for HTML email; but other MIME types are possible.