添加Nuget包

新建一个类，命名为 EmailHelper

添加相应的属性和建立构造函数，内容如下

public static EmailHelper Instance = new EmailHelper();
private string _userName;
private string _password;
private string _sender;
private string _sendEmailAddress;
private bool _isSsl = true;
private int _port;
private string _smtpServer;

private EmailHelper()
{
    _userName = "用户名";
    _password = "密码或授权码";
    _sender = "发送者（@前方的部分）";
    _sendEmailAddress = "发送的邮箱（一般和_userName一致）";
    _isSsl = false; //是否ssl加密
    _port = 25; //服务器端口
    _smtpServer = "smtp.163.com"; //网易的smtp服务器，其它的邮箱要对应的smtp服务器
}

新建一个方法，命名为 SendAsync ，具体内容如下

public async Task SendAsync(List<string> toList, List<string> ccList, List<string> bccList, string subject, string content, string contentType = null)
{
    if (toList == null || toList.Count == 0)
    {
        throw new Exception("接收地址不能为空");
    }

    if (string.IsNullOrWhiteSpace(subject))
    {
        throw new Exception("标题不能为空");
    }

    if (string.IsNullOrWhiteSpace(content))
    {
        throw new Exception("内容不能为空");
    }

    MimeMessage message = new MimeMessage();
    MailboxAddress from = new MailboxAddress(_sender, _sendEmailAddress);
    message.From.Add(from); //添加发送邮箱
  
    //添加要接收的邮箱
    foreach (string address in toList)
    {
        MailboxAddress to = new MailboxAddress(address.Split('@')[0], address);
        message.To.Add(to);
    }

    //添加要抄送的邮箱
    if (ccList != null && ccList.Count > 0)
    {
        var list = ccList.Select(p => InternetAddress.Parse(p));
        message.Cc.AddRange(list);
    }

    //添加要密送的邮箱
    if (bccList != null && bccList.Count > 0)
    {
        var list = bccList.Select(p => InternetAddress.Parse(p));
        message.Bcc.AddRange(list);
    }

    //设定邮件标题
    message.Subject = subject;

    //设定邮件的内容，内容的类型分为html格式和text格式
    BodyBuilder bodyBuilder = new BodyBuilder();
    if (contentType == "html")
    {
        bodyBuilder.HtmlBody = content;
    }
    else
    {
        bodyBuilder.TextBody = content;
    }
    message.Body = bodyBuilder.ToMessageBody();

    using (SmtpClient client = new SmtpClient())
    {
        //连接SMTP服务器
        await client.ConnectAsync(_smtpServer, _port, _isSsl);
        //验证身份
        await client.AuthenticateAsync(_userName, _password);
        //发送邮件
        await client.SendAsync(message);
        //断开服务器
        await client.DisconnectAsync(true);
    }
}

测试结果

右击SendAsync方法，在弹出的菜单中点击【创建单元测试】

生成对应的测试方案，具体内容如下

[TestClass()]
public class EmailHelperTests
{
    [TestMethod()]
    public void SendTest()
    {
        List<string> toList = new List<string>() {
                "*******@aliyun.com"
            };
        List<string> ccList = new List<string>() {
                "*******@aliyun.com"
            };
        List<string> bccList = new List<string>()
            {
                "*******@163.com"
            };
        EmailHelper.Instance.Send(toList, ccList, bccList, "测试", "测试");
    }
}

右击SendTest方法点击运行测试，如下

运行结果，如下

需要注意的是，如果在验证的时候一直在报验证失败，则看一下密码是否是对应邮箱给的验证码