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Why do I get an error by trying to send a mail with c#?

I'm trying to send an e-mail using C#, but I get this recurring error :
.
Can you explain me what's wrong with my code ?
Here it is :
SmtpClient client = new SmtpClient("smtp.gmail.com");
client.Port = 587;
client.EnableSsl = true;
client.Timeout = 100000;
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("myEmailAdress#gmail.com", "myPassword");
MailMessage msg = new MailMessage();
msg.To.Add("receiver#gmail.com");
msg.From = new MailAddress("sender#gmail.com");
msg.Subject = "My subject";
msg.Body = "My body";
client.Send(msg);
MessageBox.Show("Message sent !");

I have encountered same before.
You are getting this error because you haven't set ON on Less secure app access for sender#gmail.com as you are using Gmail SMTP port.
Reason:
Your email has no remotely access permission. You have to configure
it. Suppose you want to sent email from sender#gmail.com so you
have set that permission NO to this account.
How To Set:
You could try like below
Or can open that tab from this link directly Less secure app access
Update:
As per your comment this is for you which has working perfectly since the beginning of my career
public object SendMail(string fromEmail, string toEmail, string mailSubject, string mailBody, string senderName, string senderPass, string attacmmentLocationPath)
{
try
{
MailMessage mail = new MailMessage();
//Must be change before using
SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
mail.From = new MailAddress(fromEmail);
mail.To.Add(toEmail);
mail.Subject = mailSubject;
mail.Body = mailBody;
mail.IsBodyHtml = true;
// mail.Attachments.Add(new Attachment(#attacmmentLocationPath));
SmtpServer.Port = 587;
SmtpServer.Credentials = new System.Net.NetworkCredential(senderName, senderPass);
SmtpServer.EnableSsl = true;
SmtpServer.Send(mail);
return true;
}
catch (Exception ex)
{
return ex;
}
}
Hope that would help.

This code works for gmail, it is very similar to yours with slight differences, but if you try this, and it doesn't work for you, the issue is not the code - perhaps some other network related issue that you are going to need to fix first:
using (var msg = new MailMessage())
{
msg.From = new MailAddress("fromaddress#gmail.com");
msg.To.Add("toaddress#gmail.com");
msg.Subject = subject;
msg.Body = body;
msg.IsBodyHtml = true;
using (var smtp = new SmtpClient())
{
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.Credentials = new NetworkCredential("xxx#gmail.com","password");
smtp.Send(msg);
}
}

An SMTP client may log in using an authentication mechanism chosen among those supported by the SMTP servers.

System.Net.Mail and MailMessage not Sending Messages

I have update from an obsolete name space to System.Net.Mail - the code was supposed to be straight forward - I am having problems sending email and can't get hold of the issue
public bool send()
{
SmtpClient mailClient = new SmtpClient("my domain", 25);
MailMessage mailMessage = new MailMessage();
mailClient.EnableSsl = false;
mailMessage.From = new MailAddress("my email");
mailMessage.To.Add(sendTo.ToString());
//mailMessage.Bcc.Add(bcc.ToString());
//mailMessage.CC.Add(cc.ToString());
mailMessage.Subject = subject;
mailMessage.Body = body.ToString();
mailMessage.IsBodyHtml = true;
try
{
mailClient.Send(mailMessage);
}
catch (Exception exp)
{
exp.ToString();
return false;
}
return true;
}

using Gmail smtp server
var client = new SmtpClient();
client.Port = 587;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
if (!client.UseDefaultCredentials)
client.Credentials = new System.Net.NetworkCredential("your_email#mail.com", "your_email_pass");
client.EnableSsl = true;
client.Host = "smtp.gmail.com";
var mail = new MailMessage("from#mail.com", "to#mail.com");
mail.Subject = "test ";
mail.Body = "body message";
mail.IsBodyHtml = true;
client.Send(mail);

No Credential for the smtp server? Or you add it already.
Try
mailClient.Credentials = new NetworkCredential("username", "password"),

null value when sending email via MVC ASP.NET

I have this MVC C# App and a controller where trying to send a email, but I always got this error
this is the code in my action´controller, it doesn´t receive any model so I add specific values
[HttpPost]
public ActionResult Index2()
{
try
{
MailMessage mail = new MailMessage();
mail.To.Add("valid_email#hotmail.com");
mail.From = new MailAddress("valid_email#gmail.com");
mail.Subject = "PruebaMVC";
string Body = "PruebaMVC";
mail.Body = Body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.UseDefaultCredentials = false;
smtp.Credentials = new System.Net.NetworkCredential
("valid_email#gmail.com", "correctPWD");
smtp.EnableSsl = true;
smtp.Send(mail);
}
catch (Exception ex)
{
throw new ArgumentNullException("Exception in sendEmail:" + ex.Message);
}
return View();
}
could you please tell what is wrong?
edit: this is the exception I got

As i have used below code and it's working fine for me and i have configured smtp in web.config
ContentType HTMLType = new ContentType("text/html");
NetworkCredential cred = new NetworkCredential(StringConstant.NetworkUserName, StringConstant.NetworkPwd);
MailMessage msg = new MailMessage();
msg.BodyEncoding = System.Text.Encoding.Default;
msg.To.Add(ToEmail);
msg.Priority = System.Net.Mail.MailPriority.High;
msg.Subject = Subj;
msg.Body = strBody;
msg.IsBodyHtml = true;
if (attachement != null)
{
msg.Attachments.Add(attachement);
}
System.Net.Mail.AlternateView HTMLView = System.Net.Mail.AlternateView.CreateAlternateViewFromString(strBody, HTMLType);
msg.From = new MailAddress(StringConstant.MailFrom); // Your Email Id
SmtpClient client = new SmtpClient(StringConstant.SmtpFrom, StringConstant.SmtpPort);
client.Credentials = cred;
client.EnableSsl = true;
client.Send(msg);

Cannot send Email :( - The remote certificate is invalid according to the validation procedure

public static void SendEmail(StringBuilder sb) {
string mailbody = string.Empty;
MailMessage mail = new MailMessage();
SmtpClient smtpServer = new SmtpClient("aaaaa.com");
smtpServer.Credentials = new System.Net.NetworkCredential("username", "pwd");
//smtpServer.Port = 587;
mail.IsBodyHtml = true;
smtpServer.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
mail.BodyEncoding = System.Text.Encoding.UTF8;
smtpServer.UseDefaultCredentials = true;
smtpServer.EnableSsl = true;
mail.From = new MailAddress("bbbb.com");
mail.To.Add("ccccc.com");
mail.Subject = string.Format("test");
mailbody = sb.ToString();
mail.Body = mailbody;
smtpServer.Send(mail);
//everytime I get this error message and how to handle this, is this a security breach to bypass ssl in a company ? kindly advise the best way to handle this code and to send email.
}

Try the Unexpected “ The remote certificate is invalid according to the validation procedure.”
...tell StmpClient to ignore security with
client.EnableSsl = true;

Authentication Error in C# Send Mail App

I'm developing a simple send mail app in C#, using my CMail Server:
MailMessage mail = new MailMessage("from#mail.com", "destination#mail.com");
mail.Subject = "Sub";
mail.Body = "Hi!";
SmtpClient smtp = new SmtpClient("MyServer");
System.Net.NetworkCredential cred = new System.Net.NetworkCredential("user", "pass");
smtp.UseDefaultCredentials = false;
smtp.Credentials = cred;
smtp.Send(mail);
Obviously i ommited my account information, so, this code throws me an Authentication Exception for some reason. I first thought that the code was wrong, so i change the info to my gmail account and everything goes fine, with the only SMTP server that i having trouble is with the CMail. Is there a problem with .NET and CMail's SMTP ?
Thanks for the help and comments!

Try adding:
smtp.EnableSsl = true;

If you are using 2-step verification then you will need to add application specific password.

Full work sample
public static void sendEmail()
{
//for use GMAIL require enable -
//https://myaccount.google.com/lesssecureapps?rfn=27&rfnc=1&eid=39511352899709300&et=0&asae=2&pli=1
Console.WriteLine("START MAIL SENDER");
//Авторизация на SMTP сервере
SmtpClient Smtp = new SmtpClient("smtp.gmail.com", 587);
Smtp.EnableSsl = true;
Smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
Smtp.UseDefaultCredentials = false;
//username password
Smtp.Credentials = new NetworkCredential("rere", "rere");
//Формирование письма
MailMessage Message = new MailMessage();
Message.From = new MailAddress("rere#gmail.com");
Message.To.Add(new MailAddress("rere#gmail.com"));
Message.Subject = "test mesage";
Message.Body = "tttt body";
string file = "D:\\0.txt";
if (file != "")
{
Attachment attach = new Attachment(file, MediaTypeNames.Application.Octet);
ContentDisposition disposition = attach.ContentDisposition;
disposition.CreationDate = System.IO.File.GetCreationTime(file);
disposition.ModificationDate = System.IO.File.GetLastWriteTime(file);
disposition.ReadDate = System.IO.File.GetLastAccessTime(file);
Message.Attachments.Add(attach);
Console.WriteLine("ADD FILE [" + file + "]");
}
try
{
Smtp.Send(Message);
MessageBox.Show("SUCCESS");
}
catch { MessageBox.Show("WRONG"); }
}