Rust lettre 将电子邮件正文作为编码的 html 发送

问：

我想发送一个 base64 编码的 html，其中 lettre 作为我的电子邮件正文，这应该是可能的，但是当我在电子邮件正文中发送它时，我看到的是 onlt base64 字符串而不是我的 html 从理论上讲，如果我传递标头 content-transfer-encoding base63，它应该会自动呈现 base64，但对我来说它不起作用，这是我的代码：

    let email = Message::builder()
        .from(row.mail_from.parse()?)
        .to(row.mail_to.parse()?)
        .subject(row.mail_subject)
        .header(ContentType::TEXT_HTML)
        .header(ContentTransferEncoding::Base64)
        .body(row.mail_body)?;

    let creds = Credentials::new(smtp2go_username.to_owned(), smtp2go_password.to_owned());

    let mailer = SmtpTransport::relay(&smtp2go_server)
        ?
        .credentials(creds)
        .build();

    let result = mailer.send(&email);
    match result {
        Ok(_) => {
            info!("Email sent successfully to {} with from {}", row.mail_to, row.mail_from);
            // set the mail to sent
            let query = format!("UPDATE notifications_mail_queue SET mail_sent = 1 WHERE id = {}", row.id);
            conn.query_drop(query)?;
            // set the sent date
            let current_date = Local::now().to_rfc3339();
            let query = format!("UPDATE notifications_mail_queue SET mail_sent_date = '{}' WHERE id = {}", current_date, row.id);
            conn.query_drop(query)?;
        }
        Err(e) => {
            error!("Failed to send email to {} with from {}. Sending with {}: {:?}", row.mail_to, row.mail_from, smtp_username, e);
        }
    }