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The SMTP server requires a secure connection or the client was not authenticated for service account

I need to use a company shared account (email address is removed for privacy) to send notifications, but fails all the time. I tried all possible codes with no luck.
So, I had a little suspicious about the account. Then I used my office 365 work account, and it is working as expected. But I still need to use the shared account.
The error information is as follows:
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
[D*******8.na***11.prod.outlook.com]"
var message = new MailMessage("from", "to", "MySubject", "MyBody");
SmtpClient client = new SmtpClient("outlook.office365.com");
client.EnableSsl = true;
client.Port = 587;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("username", "password");
client.Send(message);

If your account has multi-factor authentication enabled you'll need to use an "app password" for your Office 365 account instead of a regular one that you use to log on.
See https://learn.microsoft.com/en-us/azure/active-directory/user-help/multi-factor-authentication-end-user-app-passwords for how to set one up.

SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required? [duplicate]

This question already has answers here:
mail sending with network credential as true in windows form not working
(2 answers)
Closed 7 years ago.
I'm working in windows application and C#, I am using following code to send email. The Code not works correctly in my system:
MailMessage mailmsg = new MailMessage();
SmtpClient smtpclient = new SmtpClient();
mailmsg.To.Add(txtTo.Text);
mailmsg.CC.Add(txtCC.Text);
mailmsg.Subject = txtSubj.Text;
mailmsg.From = new MailAddress("buvana#gmail.com");
mailmsg.Body = txtbody.Text;
smtpclient.Port = 587;
smtpclient.Host = "smtp.gmail.com";
smtpclient.EnableSsl = true;
smtpclient.UseDefaultCredentials = false;
smtpclient.Credentials = new NetworkCredential("buvana#gmail.com", "*********");
smtpclient.Send(mailmsg);
How to solve this problem.
Received this error:
SMTP server requires a secure connection or the client was not authenticated
The server response was: 5.5.1 Authentication Required

Your code seems to be fine, useDefaultCredentials is false, port is 587, etc.
I reckon the problem is that you need to configure Gmail to allow Less Secure Applications, following these instructions: https://support.google.com/accounts/answer/6010255?hl=en
Go to the "Less secure apps" section in My Account.
Next to "Access for less secure apps," select Turn on. (Note to Google Apps users: This setting is hidden if your administrator has locked less secure app account access.)
If you are using 2 factor authentication, you'll need to create a new app password for your application and use that to log in.

WPF application with send mail open account

i have WPF application with send mail function.
SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587);
NetworkCredential basicCredential = new NetworkCredential("user_name", "password");
smtp.Credentials = basicCredential;
smtp.EnableSsl = true;
smtp.Send(mail);
i noticed that McAfee antivirus bloks the SMTP mail by default and only after modifing McAfee configuration "prevent mass mailing worms from sending mail" property to false i could send email. is there any email account that will not be blocked with any antivirus without any configuration modification (Yahoo maybe...)?

I had the same issue and I'm afraid the only way of overcoming this is by applying a rule to your AV to allow emails sent from within your application. At least this is what I have done in the past since I couldn't find any other way.

Send email with out using network credentials

I want to send email using the SMTP client in C#. Currently I am using this code:
System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();
message.To.Add("receiver");
message.Subject = "";
message.From = new System.Net.Mail.MailAddress("sender");
message.Body = "This is a test mail";
System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("smtp.gmail.com");
smtp.Credentials = new System.Net.NetworkCredential("username", "password");
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.Send(message);
But I want to send mail with out using username and password by just providing the sender address. Is this possible in C#?
Is this possible with any SMTP server(Not google). I mean login to server anonymously and just provide your mail address.

No. Gmail does not allow anonymous senders. Thankfully.
If it was possible anyone could pretend to send emails from everyone else, rendering gmail as one big spam engine.

It depends on your server. In your code you are using Gmail and Gmail requires this.
You can set up your own SMTP server.

The SMTP server in IIS allows this per default if your application is on the same server as IIS.

In the end you'll have to send credentials if the server demands them.
You could send the credentials of the current security context if that's what you're looking for. See: http://msdn.microsoft.com/en-us/library/system.net.credentialcache.defaultnetworkcredentials.aspx.

Exchange 2007 Not Allowing Mail To Be Sent From Console App

I am trying to send email using Exchange 2007 from a console app using the following code and I get this error message in the exception that gets thrown on the Send call.
The SMTP server requires a secure
connection or the client was not
authenticated. The server response
was: 5.7.1 Client was not
authenticated
MailMessage message = new MailMessage();
message.From = new MailAddress("from#example.com");
message.To.Add("to#domain.com");
message.Subject = "test";
SmtpClient smtp = new SmtpClient(ConfigurationUtil.SMTPServer);
smtp.Credentials = new System.Net.NetworkCredential("from#example.com", "password");
smtp.Send(message);
This worked on Exchange 2003.

This ended up being an Exchange 2007 issue and had nothing to do with code.

From the error message it seems like you need to connect to Exchange via SSL.
SmtpClient smtp = new SmtpClient(ConfigurationUtil.SMTPServer, 465);
Substitute that port number for the port that your Exchange server's secure connection is listening on.