.NET SmtpClient 發送郵件完整指南：基本寄送、HTML 內容、附件、內嵌圖片、SSL/TLS、匿名轉發

核心類別

• System.Net.Mail.SmtpClient：SMTP 連線與發送
• System.Net.Mail.MailMessage：郵件內容、收件人、附件
• System.Net.Mail.Attachment：附件與內嵌資源
• System.Net.NetworkCredential：認證資訊

1. 基本文字郵件 (Web.config 組態)

<configuration>
  <appSettings>
    <add key="SmtpHost" value="smtp.gmail.com" />
    <add key="SmtpPort" value="587" />
    <add key="SmtpUser" value="your@gmail.com" />
    <add key="SmtpPass" value="app-password" /> <!-- Gmail 需應用程式密碼 -->
    <add key="EnableSsl" value="true" />
  </appSettings>
</configuration>

public static void SendText(string to, string subject, string body)
{
    var msg = new MailMessage
    {
        From = new MailAddress(ConfigurationManager.AppSettings["SmtpUser"]),
        Subject = subject,
        Body = body,
        IsBodyHtml = false
    };
    msg.To.Add(to);
    Send(msg);
}

private static void Send(MailMessage msg)
{
    using (var client = new SmtpClient
    {
        Host = ConfigurationManager.AppSettings["SmtpHost"],
        Port = int.Parse(ConfigurationManager.AppSettings["SmtpPort"]),
        EnableSsl = bool.Parse(ConfigurationManager.AppSettings["EnableSsl"]),
        Credentials = new NetworkCredential(
            ConfigurationManager.AppSettings["SmtpUser"],
            ConfigurationManager.AppSettings["SmtpPass"]),
        DeliveryMethod = SmtpDeliveryMethod.Network
    })
    {
        client.Send(msg);
    }
}

2. HTML 郵件 + 附件 + 內嵌圖片 (Inline)

public static void SendHtmlWithAttachments(string to, string subject, string htmlBody, 
    IEnumerable attachmentPaths = null, 
    Dictionary inlineImages = null) // key=ContentId, value=檔案路徑
{
    var msg = new MailMessage
    {
        From = new MailAddress("sender@example.com"),
        Subject = subject,
        Body = htmlBody,
        IsBodyHtml = true
    };
    msg.To.Add(to);

    // 一般附件
    if (attachmentPaths != null)
    {
        foreach (var path in attachmentPaths)
            msg.Attachments.Add(new Attachment(path));
    }

    // 內嵌圖片：HTML 中用 
    if (inlineImages != null)
    {
        foreach (var kvp in inlineImages)
        {
            var inline = new Attachment(kvp.Value)
            {
                ContentId = kvp.Key,
                ContentDisposition = { Inline = true, DispositionType = DispositionTypeNames.Inline }
            };
            msg.Attachments.Add(inline);
        }
    }

    Send(msg);
}

3. 使用 Stream 當附件來源 (不落地檔案)

// 例如從 FileUpload 控制項直接取得 Stream
var attachment = new Attachment(fileUpload.PostedFile.InputStream, fileUpload.PostedFile.ContentType)
{
    Name = fileUpload.FileName,
    ContentDisposition = { Inline = false } // 一般附件
};
msg.Attachments.Add(attachment);

4. 匿名/開放轉發伺服器 (內網常見)

using (var client = new SmtpClient
{
    Host = "mail.internal.corp", // 內網 SMTP
    Port = 25,                   // 無加密標準埠
    EnableSsl = false,
    UseDefaultCredentials = false,
    // Credentials = null,       // 匿名，不提供認證
    DeliveryMethod = SmtpDeliveryMethod.Network
})
{
    client.Send(msg);
}

5. 非同步發送 + 進度/完成事件 (WinForms/WPF/Console)

public static Task SendAsync(MailMessage msg)
{
    var client = new SmtpClient { /* 設定同上 */ };
    var tcs = new TaskCompletionSource