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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.

How to send an e-mail with C# through Gmail

I am getting an error when trying to send an e-mail through my web service. I have tried enabling access to less secure apps disabling 2-step verification and logging into the account via a web browser. None of the solutions on SO have worked for me. I am still getting:
Error: System.Net.Mail.SmtpException: The SMTP server requires a
secure connection or the client was not authenticated. The server
response was: 5.5.1 Authentication Required.
What can I do to fix this issue?
namespace EmailService
{
public class Service1 : IService1
{
public string SendEmail(string inputEmail, string subject, string body)
{
string returnString = "";
try
{
MailMessage email = new MailMessage();
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
// set up the Gmail server
smtp.EnableSsl = true;
smtp.Port = 587;
smtp.Credentials = new System.Net.NetworkCredential("myemail#gmail.com", "mypassword");
smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.UseDefaultCredentials = false;
// draft the email
MailAddress fromAddress = new MailAddress("cse445emailservice#gmail.com");
email.From = fromAddress;
email.To.Add(inputEmail);
email.Subject = body;
email.Body = body;
smtp.Send(email);
returnString = "Success! Please check your e-mail.";
}
catch(Exception ex)
{
returnString = "Error: " + ex.ToString();
}
return returnString;
}
}
}

Just Go here : Less secure apps , Log on using your Email and Password which use for sending mail in your c# code , and choose Turn On.
Also please go to this link and click on Continue Allow access to your Google account
also I edit it little bit :
public string sendit(string ReciverMail)
{
MailMessage msg = new MailMessage();
msg.From = new MailAddress("YourMail#gmail.com");
msg.To.Add(ReciverMail);
msg.Subject = "Hello world! " + DateTime.Now.ToString();
msg.Body = "hi to you ... :)";
SmtpClient client = new SmtpClient();
client.UseDefaultCredentials = true;
client.Host = "smtp.gmail.com";
client.Port = 587;
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Credentials = new NetworkCredential("YourMail#gmail.com", "YourPassword");
client.Timeout = 20000;
try
{
client.Send(msg);
return "Mail has been successfully sent!";
}
catch (Exception ex)
{
return "Fail Has error" + ex.Message;
}
finally
{
msg.Dispose();
}
}
If the above code don't work , try to change it like the following code :
SmtpClient client = new SmtpClient();
client.Host = "smtp.gmail.com";
client.Port = 587;
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("YourMail#gmail.com", "YourPassword");

How to fix the smtp authentication runtime error when sending email using smtp client?

I'm sending simple email messages in my application using smtp client and I was using this code before and it just works fine. Now, when I tried to run my project again from my local host computer and try to send email messages. I got a runtime error that says
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.
I don't know what just happened since it was working fine before. Now I can't send email and all I've got is this error. I could hardly troubleshoot what went wrong. How do I resolve this? Here's my code below: Thanks...
SmtpClient client = new SmtpClient();
client.Host = "smtp.gmail.com";
client.Port = 587;
client.EnableSsl = true;
client.Credentials = new System.Net.NetworkCredential(#"myemailaddress#gmail.com",#"myemailpassword");
// create message
MailMessage message = new MailMessage();
message.From = new MailAddress(TextBox4.Text);
message.To.Add(new MailAddress(TextBox1.Text));
message.Subject = TextBox2.Text;
message.Body = TextBox3.Text; //body of the message to be sent
message.BodyEncoding = System.Text.Encoding.UTF8;
message.IsBodyHtml = true;
// message.Subject = "subject";
message.SubjectEncoding = System.Text.Encoding.UTF8;
try
{
client.Send(message);
Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Alert", "alert('Mail has been successfully sent!')", true);
}
catch (SmtpException ex)
{
Response.Write(ex.Message);
}
finally
{
// Clean up.
message.Dispose();
}

Just Go here : Less secure apps , Log on using your Email and Password which use for sending mail in your c# code , and choose Turn On.
Also please go to this link and click on Continue Allow access to your Google account
also I edit it little bit :
public string sendit(string ReciverMail)
{
MailMessage msg = new MailMessage();
msg.From = new MailAddress("YourMail#gmail.com");
msg.To.Add(ReciverMail);
msg.Subject = "Hello world! " + DateTime.Now.ToString();
msg.Body = "hi to you ... :)";
SmtpClient client = new SmtpClient();
client.UseDefaultCredentials = true;
client.Host = "smtp.gmail.com";
client.Port = 587;
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Credentials = new NetworkCredential("YourMail#gmail.com", "YourPassword");
client.Timeout = 20000;
try
{
client.Send(msg);
return "Mail has been successfully sent!";
}
catch (Exception ex)
{
return "Fail Has error" + ex.Message;
}
finally
{
msg.Dispose();
}
}

How to send mail using SmtpClient in .net?

I have the following code which is not working:
public static void SendMail(string content, string title, List<string> address)
{
SmtpClient client = new SmtpClient(Server,Port);
client.Port = Port;
client.Host = Server;
client.EnableSsl = false;
client.Timeout = 10000;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential(Username, Password);
foreach(string to in address)
{
MailMessage mm = new MailMessage(From, to, title, content);
mm.BodyEncoding = UTF8Encoding.UTF8;
mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
client.Send(mm);
}
client.Dispose();
}
I am getting the following error:
Mailbox unavailable. The server response was: You must give your username and password to send mail through this service
You can see that I am passing a username and password. Why am I still getting this issue?

here i am using example of using gmail server
MailMessage mail = new MailMessage();
mail.To.Add(textBox1.Text);
mail.From = new MailAddress("Yourgmailid");
mail.Subject = "Email using Gmail";
string Body = "Hi, this mail is to test sending mail" +
"using Gmail in ASP.NET";
mail.Body = Body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Credentials = new System.Net.NetworkCredential
("Yourgmailid, "Password");
smtp.EnableSsl = true;
smtp.Send(mail);

Error whilst sending an email

I am trying to send email using c# following is my code.
try
{
System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();
msg.From = new MailAddress("kmrizwan.shahid#gmail.com");
msg.To.Add("kmrizwan.shahid#gmail.com");//Text Box for To Address
msg.Subject = "Testinng subject"; //Text Box for subject
msg.IsBodyHtml = true;
msg.Body = "testing comment is here..";//Text Box for body
msg.Priority = MailPriority.High;
SmtpClient client = new SmtpClient("smtp.gmail.com", 587);
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("kmrizwan.shahid#gmail.com", "");
client.Port = 587;
client.Host = "smtp.gmail.com";
client.EnableSsl = true;
object userstate = msg;
client.Send(msg);
}
catch (Exception ex)
{
throw ex;
}
Giving following exception

It's common for internet providers to block the port used by SMTP, except to their own outgoing mailserver. The reason is to prevent spamming.
If that is the case for you, you need to use the mail server of your internet provider instead of the GMail server.