使用发件人和收件人的电子邮件地址初始化MailMessage。应该是这样的

string from = "codeforwin@gmail.com";  //Senders email   
string to = "reciever@gmail.com";  //Receiver's email  
MailMessage msg = new MailMessage(from, to); 

阅读如何用c#发送电子邮件的完整代码片段

这样的:

mail.To = "user@hotmail.com";

应该是:

mail.To.Add(new MailAddress("user@hotmail.com"));

首先去https://myaccount.google.com/lesssecureapps，并允许不太安全的应用程序为真。

然后使用下面的代码。这下面的代码只会工作，如果你的电子邮件地址是从gmail。

static void SendEmail()
    {
        string mailBodyhtml =
            "<p>some text here</p>";
        var msg = new MailMessage("from@gmail.com", "to1@gmail.com", "Hello", mailBodyhtml);
        msg.To.Add("to2@gmail.com");
        msg.IsBodyHtml = true;
        var smtpClient = new SmtpClient("smtp.gmail.com", 587); //**if your from email address is "from@hotmail.com" then host should be "smtp.hotmail.com"**
        smtpClient.UseDefaultCredentials = true;
        smtpClient.Credentials = new NetworkCredential("from@gmail.com", "password");
        smtpClient.EnableSsl = true;
        smtpClient.Send(msg);
        Console.WriteLine("Email Sent Successfully");
    }

到2017年3月，这对我来说是有效的。 从上面的解决方案开始:“终于开始工作了:)”，一开始并没有工作。

SmtpClient client = new SmtpClient();
client.Port =  587;
client.Host = "smtp.gmail.com";
client.EnableSsl = true;
client.Timeout = 10000;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("<me>@gmail.com", "<my pw>");
MailMessage mm = new MailMessage(from_addr_text, to_addr_text, msg_subject, msg_body);
mm.BodyEncoding = UTF8Encoding.UTF8;
mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

client.Send(mm);

MailKit是一个开源的跨平台。net邮件客户端库，它基于MimeKit并针对移动设备进行了优化。 它拥有比System.Net.Mail更好的更多高级功能 微软通过MimeKit支持TNEF。

从这里下载nuget包。

请看这个例子，你可以发送邮件

            MimeMessage mailMessage = new MimeMessage();
            mailMessage.From.Add(new MailboxAddress(senderName, sender@address.com));
            mailMessage.Sender = new MailboxAddress(senderName, sender@address.com);
            mailMessage.To.Add(new MailboxAddress(emailid, emailid));
            mailMessage.Subject = subject;
            mailMessage.ReplyTo.Add(new MailboxAddress(replyToAddress));
            mailMessage.Subject = subject;
            var builder = new BodyBuilder();
            builder.TextBody = "Hello There";            
            try
            {
                using (var smtpClient = new SmtpClient())
                {
                    smtpClient.Connect("HostName", "Port", MailKit.Security.SecureSocketOptions.None);
                    smtpClient.Authenticate("user@name.com", "password");

                    smtpClient.Send(mailMessage);
                    Console.WriteLine("Success");
                }
            }
            catch (SmtpCommandException ex)
            {
                Console.WriteLine(ex.ToString());              
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());                
            }

邮件。寄和寄。From是只读的。将它们移动到构造函数中。

using System.Net.Mail;

...

MailMessage mail = new MailMessage("you@yourcompany.example", "user@hotmail.com");
SmtpClient client = new SmtpClient();
client.Port = 25;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Host = "smtp.gmail.com";
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body";
client.Send(mail);