java - 使用 JavaMail 发送带有内联图像的 HTML 电子邮件 - 图像加载速度慢？

Question

当我使用 JavaMail 发送 HTML 电子邮件，并在 HTML 中包含内嵌图像时，在 Gmail 或 Yahoo 中阅读时，图像需要 2-3 秒才能加载。

我使用的图像是一个小的 .png，大小约为 200 字节。

这是代码:

import java.io.File;
import java.util.Properties;

import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;


public class EmailTest 
{   
    static String from = "Your_Gmail_Account_Name";  // (e.g., "name" if your account is "name@gmail.com"
    static String password = "Your_Gmail_Password";    
    static String to = "Send_Here@gmail.com";
    static String subject = "test";
    static String body = "<h1>The image in this e-mail is slow to load.</h1><img src=\"cid:my-image\">"; 
    static String host = "smtp.gmail.com";

    public static void main(String[] args)
    {
        Properties properties = System.getProperties();

        properties.put("mail.smtp.starttls.enable", "true");  
        properties.put("mail.smtp.host", host);
        properties.put("mail.smtp.user", from);
        properties.put("mail.smtp.password", password);
        properties.put("mail.smtp.port", "587");
        properties.put("mail.smtp.auth", "true");

        Session session = Session.getDefaultInstance(properties);

        MimeMessage message = new MimeMessage(session);

        try 
        {
            message.setFrom(new InternetAddress(from));    
            InternetAddress toAddress = new InternetAddress(to);
            message.addRecipient(Message.RecipientType.TO, toAddress);
            message.setSubject(subject);

            Multipart multipart = new MimeMultipart("related");

            MimeBodyPart htmlPart = new MimeBodyPart();
            htmlPart.setContent(body, "text/html");
            multipart.addBodyPart(htmlPart);

            MimeBodyPart imagePart = new MimeBodyPart();
            DataSource ds = new FileDataSource(new File("src\\icon.png"));
            imagePart.setDataHandler(new DataHandler(ds));
            imagePart.setHeader("Content-ID", "<my-image>");
            multipart.addBodyPart(imagePart);

            message.setContent(multipart);

            Transport transport = session.getTransport("smtp");

            try 
            {
                transport.connect(host, from, password);
                transport.sendMessage(message, message.getAllRecipients());
                transport.close();

                System.out.println("e-mail sent.");
            } 
            catch (MessagingException e) 
            {
                e.printStackTrace();
            }
        }
        catch (AddressException e) 
        {
            e.printStackTrace();
        }
        catch (MessagingException e) 
        {
            e.printStackTrace();
        }
    } 
}

有人知道为什么图片加载这么慢吗？

更新:

当在 Thunderbird 中阅读上述代码生成的电子邮件时，图像甚至不会加载，而只会显示为附件。

但是如果我删除这一行:

imagePart.setHeader("Content-ID", "<my-image>");

并将其替换为以下两行:

imagePart.addHeader("Content-ID", "<my-image>");
imagePart.addHeader("Content-Type", "image/png");

然后图像实际上在 Thunderbird 中加载，并且立即加载。

但是，Gmail 和 Yahoo 中的图像加载速度仍然很慢。

Answer 1

也许他们正在运行病毒扫描程序？尝试雷鸟。