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smtp fails to send mail sometimes in c#

I have gone through some questions on this topic. All the answers relate when sending email fails all the time. In my case, it fails only sometimes with exception message:
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM...
If I try second time it works. I'm using the following configuration.
using (MailMessage mail = new MailMessage())
{
mail.From = new MailAddress("emailid", "displayname");
mail.To.Add("TOAddress");
mail.Subject = subject1;
mail.Body = body1;
mail.IsBodyHtml = true;
using (SmtpClient smtp = new SmtpClient("Outlook.office365.com", 587))
{
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential("emailid", "password");
smtp.EnableSsl = true;
smtp.Send(mail);
}
}
checked a similar question here , given solutions not working.

Try this(second answer): Gmail Error :The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required
I used this code to send the email:
MailMessage msg = new MailMessage();
msg.From = new MailAddress("sender#gmail.com");
msg.To.Add("receiver#gmail");
msg.Subject = "Hello";
msg.Body = "Test";
SmtpClient smt = new SmtpClient();
smt.Host = "smtp.gmail.com";
System.Net.NetworkCredential ntcd = new NetworkCredential();
ntcd.UserName = "sender#gmail.com";
ntcd.Password = "senderPassword";
smt.Credentials = ntcd;
smt.EnableSsl = true;
smt.Port = 587;
smt.Send(msg);
Also check if your virus scanner doens't block your email from sending.

The server response was: 5.7.3 Requested action aborted; user not authenticated

I want to send emails using C# for an MVC project and hotmail, I have tried port 25 and 587, using my own credentials, but every time I get '5.7.3 user not authenticated', if I change SSL to false, then I get '5.7.0, Must issue a STARTTLS command first'.
There are 2 other questions with the same problem but one of the answers is old (points me to a setting that doesn't exist in outlook.com) and the other one didn't solve it (changing ssl to false).
This is my code (of course I removed my email and password), question is, what else can I do to get ride of the 5.7.3, am I missing something?
MailMessage mail = new MailMessage(<email>, <email>);
SmtpClient client = new SmtpClient("smtp-mail.outlook.com");
client.Port = 587;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential(<email>, <pass>);
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body";
client.EnableSsl = true;
client.Send(mail);

This code Works normally on my computer!
There are two issues that you should look at:
If you enabled the use of external applications in hotmail
Check the https://account.microsoft.com/account link if your computer
is locked.
My code:
MailMessage mail = new MailMessage("from", "to");
SmtpClient client = new SmtpClient();
client.EnableSsl = true;
client.Host = "smtp-mail.outlook.com";
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("email", "password");
client.Port = 587;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
mail.Subject = "test";
mail.Body = "test";
client.Send(mail);

smtp failure sending Email. How to solve?

MailMessage message = new MailMessage();
message.From = new MailAddress("MyMailAddress");
message.To.Add("DestinationMailAddress");
message.CC.Add("CCMailAddress");
message.Subject = "This is Subject";
message.SubjectEncoding = System.Text.Encoding.UTF8;
message.BodyEncoding = System.Text.Encoding.UTF8;
message.Body = "This is a test e-mail message sent by an application. ";
SmtpClient client = new SmtpClient();
client.Port = 587;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = true;
client.EnableSsl = true;
client.Host = "smtp.gmail.com";
client.Send(message);
This my code. The error was
Failure sending email.
Inner Exception:
{"Unable to connect to the remote server"} {"A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 74.125.68.109:587"} Errorcode:10060

Your code uses gmail as your SMTP:
client.Host = "smtp.gmail.com";
Yet you specify your SMTP login credentials to be your windows account:
client.UseDefaultCredentials = true;
If gmail is in fact your SMTP, you need to set this to false and then provide your gmail login credentials.
So your code should look like:
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("yourusername", "yourpassword");
It also looks like you might have a firewall issue as well
{"Unable to connect to the remote server"} {"A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 74.125.68.109:587"} Errorcode:10060
Make sure that the application has access to the internet and the necessary ports are open to reach gmail. Also make sure that on your gmail account, less ssecure applications have been allowed, and that, if you are using 2-factor authentication, that you have created an application-specific password for the account and are using that to connect.
How to enable less secure apps: https://support.google.com/accounts/answer/6010255?hl=en
You might also need to change how your MailAddress.To is populated:
message.To.Add(new MailAddress("DestinationMailAddress"));

Since the functionality is wrapped you should use the inner exception message to get the detailed error message. The failure reason will be right there!
public static void Main()
{
try {
SendMail();
}
catch(Exception e) {
if (e.InnerException != null)
Console.WriteLine("Inner exception: {0}", e.InnerException);
}
}

Make these changes
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("yourusername", "yourpassword");
And this too
http://docs.helpscout.net/article/120-smtp-settings
Full working code
protected void SendMail()
{
MailMessage msg = new MailMessage();
System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient();
try
{
msg.Subject = "Add Subject";
msg.Body = "Add Email Body Part";
msg.From = new MailAddress("Valid Email Address");
msg.To.Add("Valid Email Address");
msg.IsBodyHtml = true;
client.Host = "smtp.gmail.com";
System.Net.NetworkCredential basicauthenticationinfo = new System.Net.NetworkCredential("Valid Email Address", "Password");
client.Port = int.Parse("587");
client.EnableSsl = true;
client.UseDefaultCredentials = false;
client.Credentials = basicauthenticationinfo;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Send(msg);
}
catch (Exception ex)
{
log.Error(ex.Message);
}
}

SMTP Mail with ASP.net Error 5.5.1 Authentication Required

Good day, I'm a beginner from using ASP.net and SMTP Mailer
Heres my Question, I always encounter this Error when i send email from my local
and searched and tried the solutions around the net but not so lucky,
I hope someone point out what codes do i need and where i encounter this errror
Message = "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at"
Heres my Code:
protected void btnSendEmail_Click(object sender, EventArgs e)
{
// System.Web.Mail.SmtpMail.SmtpServer is obsolete in 2.0
// System.Net.Mail.SmtpClient is the alternate class for this in 2.0
SmtpClient smtpClient = new SmtpClient();
MailMessage message = new MailMessage();
try
{
MailAddress fromAddress = new MailAddress(txtEmail.Value, txtName.Value);
smtpClient.Credentials = new System.Net.NetworkCredential("myUser#gmail", "password");
// You can specify the host name or ipaddress of your server
// Default in IIS will be localhost
smtpClient.Host = "smtp.gmail.com";
smtpClient.EnableSsl = true;
//Default port will be 25
smtpClient.Port = 25;
smtpClient.UseDefaultCredentials = false;
//From address will be given as a MailAddress Object
message.From = fromAddress;
// To address collection of MailAddress
message.To.Add("myEmail#gmail.com");
message.Subject = txtSubject.Value;
// CC and BCC optional
// MailAddressCollection class is used to send the email to various users
// You can specify Address as new MailAddress("admin1#yoursite.com")
message.CC.Add("myEmail#gmail.com");
// You can specify Address directly as string
message.Bcc.Add(new MailAddress("myEmail#gmail.com"));
//Body can be Html or text format
//Specify true if it is html message
message.IsBodyHtml = false;
// Message body content
message.Body = txtaMessage.Value;
message.BodyEncoding = System.Text.Encoding.UTF8;
message.HeadersEncoding = System.Text.Encoding.UTF8;
// Send SMTP mail
smtpClient.Send(message);
lblSuccess.Text = "Email successfully sent.";
}
catch (Exception ex)
{
lblSuccess.Text = "Send Email Failed.";
}
}

i tried to make a simple codes for sending email try this
MailMessage mm = new MailMessage();
mm.From = new MailAddress("fromEmail");
mm.To.Add("toEmail");
mm.CC.Add("ccEmail");
mm.Subject = "StringSubject";
mm.Body = "BodySubject";
mm.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
NetworkCred.UserName = "UsernameString";
NetworkCred.Password = "PasswordString";
smtp.UseDefaultCredentials = true;
smtp.Credentials = NetworkCred;
smtp.Port = 587;
try
{
smtp.Send(mm);
}
catch (Exception)
{
}

Try this reference ASP Simple SMTP for C# and VB it helps me a lot for may smtp problem

Please have a look on google support team, what they are saying regarding sending mail from application.
https://support.google.com/mail/answer/78775?hl=en
Also following link can help you.
Gmail Error :The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required

Goto Account of Gmail , then select
Connected apps & sites
Allow less secure apps: ON(if this is off you cannot send mails through apps,or your websites )

Problems sending e-mail in web app

I have been trying for 2 days to get my ASP.NET webforms application to send an e-mail.
I have tried this using both outlook and gmail. I got the smtp information for both from this tutorial:
When I try using port 587 in the example below I get an error saying:
An exception of type 'System.Net.Mail.SmtpException' occurred in System.dll but was not handled in user code
Additional information: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.
When I try using port 465 in the example below:
My application just hangs forever and my page never gets back my email function which is in the PageLoad.
A couple of things to note just incase one of these is messing me up:
I am using a standard VS2013 dev environment (running my web app in debug mode using F5)
I am using the same e-mail address for from, to, and gmail login credentials)
My ISP does not block these ports. (comcast cable)
I even went to the google DisplayUnlockCaptcha page to Allow access to my account
protected void SendEmail()
{
string EmailAddress = "myemail#gmail.com";
using (MailMessage mailMessage = new MailMessage(EmailAddress, EmailAddress))
{
mailMessage.Subject = "This is a test email";
mailMessage.Body = "This is a test email. Please reply if you receive it.";
SmtpClient smtpClient = new SmtpClient();
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
smtpClient.EnableSsl = true;
smtpClient.Host = "smtp.gmail.com";
smtpClient.Port = 587;
smtpClient.Credentials = new System.Net.NetworkCredential()
{
UserName = EmailAddress,
Password = "password"
};
smtpClient.UseDefaultCredentials = false;
smtpClient.Send(mailMessage);
}
}

This code should work fine for you
protected void SendEmail()
{
string EmailAddress = "myemail#gmail.com";
MailMessage mailMessage = new MailMessage(EmailAddress, EmailAddress);
mailMessage.Subject = "This is a test email";
mailMessage.Body = "This is a test email. Please reply if you receive it.";
SmtpClient smtpClient = new SmtpClient();
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
smtpClient.EnableSsl = true;
smtpClient.Host = "smtp.gmail.com";
smtpClient.Port = 587;
smtpClient.Credentials = new System.Net.NetworkCredential()
{
UserName = EmailAddress,
Password = "password"
};
smtpClient.UseDefaultCredentials = false;
smtpClient.Send(mailMessage);
}
You'll need to set the delivery mode otherwise gmail will return an error
EDIT:
Throwing an 'using' around 'MailMessage' might also be a smart thing to do

It turns out that it is because of a GMail security setting.
https://www.google.com/settings/security/lesssecureapps
You have to enable access for less secure apps.

public void sendEmail(string body)
{
if (String.IsNullOrEmpty(email))
return;
try
{
MailMessage mail = new MailMessage();
mail.To.Add(email);
mail.To.Add("xxx#gmail.com");
mail.From = new MailAddress("yyy#gmail.com");
mail.Subject = "sub";
mail.Body = body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
smtp.Credentials = new System.Net.NetworkCredential
("yyy#gmail.com", "pw"); // ***use valid credentials***
smtp.Port = 587;
//Or your Smtp Email ID and Password
smtp.EnableSsl = true;
smtp.Send(mail);
}
catch (Exception ex)
{
print("Exception in sendEmail:" + ex.Message);
}
}``
http://www.c-sharpcorner.com/UploadFile/47548d/how-to-send-bulk-email-using-Asp-Net/