[Route("api/[controller]")]
[ApiController]
public class MailController : ControllerBase
{
    /// <summary>
    /// 發送郵件
    /// </summary>
    [HttpPost("send")]
    public IActionResult PostEmail([FromBody] Mail mailDto)
    {
        if (!ModelState.IsValid)
        {
            // 返回驗證錯誤信息
            BadRequest(ModelState);
        }
        SMTPSendHelper.SendEmailBySMTP(mailDto);
        return new JsonResult(new
        {
            message = "發送成功",
            code = 0,
            success = true
        });
    }
}
/// <summary>
/// 郵件發送幫助類
/// </summary>
public class SMTPSendHelper
{
    /// <summary>
    /// 發送郵件
    /// </summary>
    public static void SendEmailBySMTP(Mail mail)
    {
        //實例化一個發送郵件類
        MailMessage mailMsg = new MailMessage();
        //發件人郵箱地址
        mailMsg.From = new MailAddress(mail.senderEmail);
        //收件人郵箱地址
        mailMsg.To.Add(new MailAddress(mail.receiverEmail));
        //郵件標題的編碼格式
        mailMsg.SubjectEncoding = Encoding.UTF8;
        //郵件標題
        mailMsg.Subject = mail.emailTitle;
        //郵件內容的編碼格式
        mailMsg.BodyEncoding = Encoding.UTF8;
        //郵件內容
        mailMsg.Body = mail.emailContent;
        //是否是html郵件
        mailMsg.IsBodyHtml = true;
        //郵件優先級
        mailMsg.Priority = MailPriority.High;
        //實例化一個SmtpClient類
        System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient();
        //設置郵件服務名，這里使用的是QQ郵箱，所以是smtp.qq.com, 若使用163郵箱，則是 smtp.163.com
        client.Host = mail.smtpService;
        //設置郵件端口
        client.Port = 587;
        //使用安全加密連接
        client.EnableSsl = true;
        //不和請求一塊發送
        client.UseDefaultCredentials = false;
        //驗證發件人身份（發件人郵箱，郵箱生成的授權碼）
        client.Credentials = new NetworkCredential(mail.senderEmail, mail.authCode);
        //發送
        client.Send(mailMsg);
    }
}
/// <summary>
/// 郵件參數類 
/// </summary>
public class Mail
{
    /// <summary>
    /// 發件人郵箱
    /// </summary>
    public string senderEmail { set; get; } = "471812366@qq.com";
    /// <summary>
    /// 發件人郵箱授權碼
    /// </summary>
    public string authCode { set; get; } = "igghkcotbwfycadh";
    /// <summary>
    /// 收件人郵箱
    /// </summary>
    [Required(ErrorMessage = "收件人郵箱是必填項")]
    public string? receiverEmail { set; get; }
    /// <summary>
    /// 郵件標題
    /// </summary>
    [Required(ErrorMessage = "郵件標題是必填項")]
    public string? emailTitle { set; get; }
    /// <summary>
    /// 郵件內容
    /// </summary>
    [Required(ErrorMessage = "郵件內容是必填項")]
    public string? emailContent { set; get; }
    /// <summary>
    /// 郵件服務名：smtp.qq.com
    /// </summary>
    public string smtpService { set; get; } = "smtp.qq.com";
}

//測試發送郵件
var mail = new Mail()
{
    receiverEmail = "504968510@qq.com",
    emailTitle = "測試郵件",
    emailContent = "測試郵件內容：htttp://www.baidu.com"
};

{
	"receiverEmail":"504968510@qq.com",
	"emailTitle":"測試標題",
	"emailContent":"測試郵件內容：<b>htttp://www.baidu.com</b>"
}