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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.

Error on sending email from C# code

When I send mail to my gmail account, it shows below error.
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Authentication required...
code I am using is below
MailMessage m = new MailMessage();
SmtpClient sc = new SmtpClient();
try
{
m.From = new MailAddress("me#gmail.com");
m.To.Add("me#gmail.com");
m.Subject = "This is a Test Mail";
m.IsBodyHtml = true;
m.Body = "test gmail";
sc.Host = "smtp.gmail.com";
sc.Port = 587;
sc.Credentials = new System.Net.NetworkCredential("me#gmail.com", "passward");
sc.UseDefaultCredentials = true;
sc.EnableSsl = true;
sc.Send(m);
Response.Write("Email Send successfully");
}
catch (Exception ex)
{
Response.Write(ex.Message);
}

Just tried your code, had to fiddle with a couple things but was sent this. Funny because I have done this previously using Gmail smtp (couple years back). But it looks like they are now verifying apps that use their platform.
Either use another smtp server that you are signed up to, or use your own. (there must be a test one that is available online??). Pretty sure sendgrid do a free trial.

using System.Net;
using System.Net.Mail;
string smtpAddress = "smtp.mail.yahoo.com";
int portNumber = 587;
bool enableSSL = true;
string emailFrom = "email#yahoo.com";
string password = "abcdefg";
string emailTo = "someone#domain.com";
string subject = "Hello";
string body = "Hello, I'm just writing this to say Hi!";
using (MailMessage mail = new MailMessage())
{
mail.From = new MailAddress(emailFrom);
mail.To.Add(emailTo);
mail.Subject = subject;
mail.Body = body;
mail.IsBodyHtml = true;
// Can set to false, if you are sending pure text.
mail.Attachments.Add(new Attachment("C:\\SomeFile.txt"));
mail.Attachments.Add(new Attachment("C:\\SomeZip.zip"));
using (SmtpClient smtp = new SmtpClient(smtpAddress, portNumber))
{
smtp.Credentials = new NetworkCredential(emailFrom, password);
smtp.EnableSsl = enableSSL;
smtp.Send(mail);
}
}
Please try this this should work for you
Thank you

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();
}
}

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.

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"); }
}