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C#. SMTP The server response was: 5.7.0 Authentication Required

Error: The SMTP server requires a secure connecton or the client was not authenticated. The server
response was: 5.7.0 Authentication Required. Learn more at.
private void SendMail(string email, string firstname)
{
MailAddress from = new MailAddress($"{email}", $"{firstname}");
MailAddress to = new MailAddress($"{email}");
MailMessage m = new MailMessage(from, to);
m.Subject = "TEST";
m.Body = "<h2>TEST</h2>";
m.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587);
smtp.Credentials = new NetworkCredential("jewralyapp#mail.ru", "password");
smtp.EnableSsl = true;
smtp.Send(m);
}
I want to send a letter to the mail, but the error that I indicated above comes out, here is the code, can you tell me what is wrong?

I see that your email is #mail.ru and the server is gmail, is that correct? I think you need to use your gmail account.
You also need enable weaker credentials on your gmail account. Forgot what they actually call it but it's on gmail security setting.

I had also faced the same problem.
Your SmtpClient is gmail, but you are using "jewralyapp#mail.ru"!!!
Make sure to set "Less secure app access" to ON in your Gmail security setting.
In my case, the problem was solved.

SMTP 5.7.57 error when trying to send email via Office 365

I'm trying to set up some code to send email via Office 365's authenticated SMTP service:
var _mailServer = new SmtpClient();
_mailServer.UseDefaultCredentials = false;
_mailServer.Credentials = new NetworkCredential("test.user#mydomain.com", "password");
_mailServer.Host = "smtp.office365.com";
_mailServer.TargetName = "STARTTLS/smtp.office365.com"; // same behaviour if this lien is removed
_mailServer.Port = 587;
_mailServer.EnableSsl = true;
var eml = new MailMessage();
eml.Sender = new MailAddress("test.user#mydomain.com");
eml.From = eml.Sender;
eml.to = new MailAddress("test.recipient#anotherdomain.com");
eml.Subject = "Test message";
eml.Body = "Test message body";
_mailServer.Send(eml);
This doesn't appear to be working, and I'm seeing an exception:
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
at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
I've tried enabling network tracing and it appears that secure communications are established (for example, I see a line in the log for the "STARTTLS" command, and later there's a line in the log "Remote certificate was verified as valid by the user.", and the following Send() and Receive() data is not readable as plain text, and doesn't appear to contain any TLS/SSH panics)
I can use the very same email address and password to log on to http://portal.office.com/ and use the Outlook email web mail to send and read email, so what might be causing the authentication to fail when sending email programmatically?
Is there any way to additionally debug the encrypted stream?

In my case after I tried all this suggestion without luck, I contacted Microsoft support, and their suggestion was to simply change the password.
This fixed my issue.
Note that the password wasn't expired, because I logged on office365 with success, however the reset solved the issue.
Lesson learned: don't trust the Office 365 password expiration date, in my case the password would be expired after 1-2 months, but it wasn't working.
This leaded me to investigate in my code and only after a lot of time I realized that the problem was in the Office365 password that was "corrupted" or "prematurely expired".
Don't forget every 3 months to "refresh" the password.

To aid in debugging, try temporarily switching to MailKit and using a code snippet such as the following:
using System;
using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;
namespace TestClient {
class Program
{
public static void Main (string[] args)
{
var message = new MimeMessage ();
message.From.Add (new MailboxAddress ("", "test.user#mydomain.com"));
message.To.Add (new MailboxAddress ("", "test.recipient#anotherdomain.com"));
message.Subject = "Test message";
message.Body = new TextPart ("plain") { Text = "This is the message body." };
using (var client = new SmtpClient (new ProtocolLogger ("smtp.log"))) {
client.Connect ("smtp.office365.com", 587, SecureSocketOptions.StartTls);
client.Authenticate ("test.user#mydomain.com", "password");
client.Send (message);
client.Disconnect (true);
}
}
}
}
This will log the entire transaction to a file called "smtp.log" which you can then read through and see where things might be going wrong.
Note that smtp.log will likely contain an AUTH LOGIN command followed by a few commands that are base64 encoded (these are your user/pass), so if you share the log, be sure to scrub those lines.
I would expect this to have the same error as you are seeing with System.Net.Mail, but it will help you see what is going on.
Assuming it fails (and I expect it will), try changing to SecureSocketOptions.None and/or try commenting out the Authenticate().
See how that changes the error you are seeing.

Be sure you're using the actual office365 email address for the account. You can find it by clicking on the profile button in Outlook365. I wrestled with authentication until I realized the email address I was trying to use for authentication wasn't the actual mailbox email account. The actual account email may have the form of: account#company.onmicrosoft.com.

We got ours working by converting the mailboxes (from address) from "shared" to "regular". Before this change, my application quit sending email when we migrated from Gmail to Office 365. No other code changes were required, besides setting the host to smtp.office365.com.

Please check below code I have tested to send email using Exchange Online:
MailMessage msg = new MailMessage();
msg.To.Add(new MailAddress("YourEmail#hotmail.com", "XXXX"));
msg.From = new MailAddress("XXX#msdnofficedev.onmicrosoft.com", "XXX");
msg.Subject = "This is a Test Mail";
msg.Body = "This is a test message using Exchange OnLine";
msg.IsBodyHtml = true;
SmtpClient client = new SmtpClient();
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("XXX#msdnofficedev.onmicrosoft.com", "YourPassword");
client.Port = 587; // You can use Port 25 if 587 is blocked
client.Host = "smtp.office365.com";
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.EnableSsl = true;
try
{
client.Send(msg);
}
catch (Exception ex)
{
}
Port (587) was defined for message submission. Although port 587 doesn't mandate requiring STARTTLS, the use of port 587 became popular around the same time as the realisation that SSL/TLS encryption of communications between clients and servers was an important security and privacy issue.

In my case my problem was not related to the code but something to do with the Exchange mailbox. Not sure why but this solved my problem:
Go to the exchange settings for that user's mailbox and access Mail Delegation
Under Send As, remove NT AUTHORITY\SELF and then add the user's account.
This gives permissions to the user to send emails on behalf of himself. In theory NT AUTHORITY\SELF should be doing the same thing but for some reason that did not work.
Source: http://edudotnet.blogspot.com.mt/2014/02/smtp-microsoft-office-365-net-smtp.html

I got this same error while testing, using my own domain email account during development. The issue for me seemed related to the MFA (Multi Factor Authentication) that's enabled on my account. Switching to an account without MFA resolved the issue.

I had this issue since someone had enabled Security defaults in Azure.
This disables SMTP/Basic authentication. It's clearly stated in the documentation, but it's not evident by the error message, and you have to have access to the account to find out.
https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/concept-fundamentals-security-defaults
It's possible to enable it per account.
https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission

You need change the credentials function. Here is the substitution you need to make:
change
-*_mailServer.Credentials = new NetworkCredential("test.user#mydomain.com", "password");*
for this
-*_mailServer.Credentials = new NetworkCredential("test.user#mydomain.com", "password", "domain");*

In my case, password was expired.I just reset password and its started working again

Mailbox unavailable. The server response was: relay not permitted

I am sending emails via an external SMTP server. Sending the email is handled with this code:
try
{
System.Net.Mail.MailMessage mail = new System.Net.Mail.MailMessage();
mail.From = new MailAddress(froma);
mail.To.Add(toc);
mail.Subject = subject;
mail.IsBodyHtml = true;
mail.Body = body;
SmtpClient smtp = new SmtpClient(ClientServer);
DataSet ds = new DataSet();
int retCode = Email.getSmtp(ref ds, DatabaseName);
string User="";
string Password="";
if (ds.Tables["Value"].Rows.Count > 0)
{
User = ds.Tables["Value"].Rows[0]["UserName"].ToString();
Password = ds.Tables["Value"].Rows[0]["PasswordName"].ToString();
}
else
{
MessageBox.Show("Invalid SMTP settings!");
return;
}
smtp.UseDefaultCredentials = false;
smtp.Credentials = new System.Net.NetworkCredential(User, Password);
smtp.EnableSsl = false;
smtp.Send(mail);
}
catch (System.Web.HttpException exHttp)
{
System.Console.WriteLine("Exception occurred:" + exHttp.Message);
}
Testing this code on my server, with my own SMTP server on the same network, this returns all my emails. However, using an external SMTP server causes the error:
Mailbox unavailable. The server response was: relay not permitted.
I have read around and it appears that the admin for SMTP must allow relays for my server. However, using the authentication credentials provided, I can't seem to connect, and am still receiving the relay error.

Yes, it sounds like the external server that you are using is not allow relay. Even if you have the proper authentication credentials, you will not be able to send the email because the relay function is still disabled. Are you the admin of this external server? If you are then you can enable it. This LINK HERE explains how to set up SMTP and the relay. If you are not the admin of this external server, then you will have to contact who is so they can enable the SMTP and relay for you.
It sounds like the server on your network has SMTP installed and the relay is set up properly since you are able to send. I had to install SMTP and configure the relay on all three of servers here (development box, staging box, and production box) to send emails. I hope this information helps.

I had the same error and commented out :
//smtp.UseDefaultCredentials = true;
And the email was sent successfully.

How to send a Secure e-mail using SMTP

I am currently using Google Apps to send SMTP e-mails. If my project deploys some of the information that i am going to be sending will be confidential and i would like to make sure the transmission is secure. Can anyone please let me know what i need to do to ensure that i send a safe e-mail using smtp through the google apps smtp server? smtp.google.com.
Any help greatly appreciated.
From what I have been told i need to force Https and have a SSL cert in order to do this. I don't know if this is true?

You Can Use 'smtp.EnableSsl = true' For Enable SSL for security.
MailMessage mail = new MailMessage();
mail.To.Add("" + to + "");
mail.From = new MailAddress("" + from + "");
mail.Subject = "Email using Gmail";
string Body = "Hi, this mail is to test sending mail" +
"";
mail.Body = Body;
mail.IsBodyHtml = true;
Attachment at = new Attachment(Server.MapPath("~/ExcelFile/TestCSV.csv"));
mail.Attachments.Add(at);
mail.Priority = MailPriority.High;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
smtp.Credentials = new System.Net.NetworkCredential(""+ username +"", ""+ password +"");
smtp.EnableSsl = true;
smtp.Port = 587;
smtp.Send(mail);

To enforce Network Security you have to use SSL. to enforce security of the data going from your webserver to mail server you need to send your mail over SSL. and to secure the HTTP request that triggers the mail action you need to enforce SSL over HTTP.
But the question is Security in what context ? If you need network security to ensure a 3rd party cannot eavesdrop or manipulate then SSL is your way to go.

How to Send Email via Yandex SMTP (C# ASP.NET)

Formerly, I used my server as mail host and was sending emails via my own host. Now, I use Yandex as my mail server. I'm trying to send emails via Yandex SMTP. However, I could not achieve it. I get "the operation has timed out" message every time. I'm able to send & receive email with the same settings when I use Thunderbird. Hence, there is no issue with the account. I appreciate your guidance. You can see my code below:
EmailCredentials credentials = new EmailCredentials();
credentials.Domain = "domain.com";
credentials.SMTPUser = "email#domain.com";
credentials.SMTPPassword = "password";
int SmtpPort = 465;
string SmtpServer = "smtp.yandex.com";
System.Net.Mail.MailAddress sender = new System.Net.Mail.MailAddress(senderMail, senderName, System.Text.Encoding.UTF8);
System.Net.Mail.MailAddress recipient = new System.Net.Mail.MailAddress(recipientEmail, recipientName, System.Text.Encoding.UTF8);
System.Net.Mail.MailMessage email = new System.Net.Mail.MailMessage(sender, recipient);
email.BodyEncoding = System.Text.Encoding.UTF8;
email.SubjectEncoding = System.Text.Encoding.UTF8;
System.Net.Mail.AlternateView plainView = System.Net.Mail.AlternateView.CreateAlternateViewFromString(System.Text.RegularExpressions.Regex.Replace(mailBody, #"<(.|\n)*?>", string.Empty), null, MediaTypeNames.Text.Plain);
System.Net.Mail.AlternateView htmlView = System.Net.Mail.AlternateView.CreateAlternateViewFromString(mailBody, null, MediaTypeNames.Text.Html);
email.AlternateViews.Clear();
email.AlternateViews.Add(plainView);
email.AlternateViews.Add(htmlView);
email.Subject = mailTitle;
System.Net.Mail.SmtpClient SMTP = new System.Net.Mail.SmtpClient();
SMTP.Host = SmtpServer;
SMTP.Port = SmtpPort;
SMTP.EnableSsl = true;
SMTP.Credentials = new System.Net.NetworkCredential(credentials.SMTPUser, credentials.SMTPPassword);
SMTP.Send(email);

After so many trials & errors, I have found how to make it work. I have made the following changes on the code posted in the question:
Set SmtpPort = 587
Added the following 2 lines of code:
SMTP.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
SMTP.UseDefaultCredentials = false;
Additional note:
I use Azure server. I realized later that I did not configure the smtp endpoint for port 465. That being said, I had to add the 2 lines of code above in order to make email delivery work, just changing the port was not enough. My point is it is worth to check the defined ports on Azure and firewall before doing anything further.
I was able to make my code work by getting help from #Uwe and also #Dima-Babich, #Rail who posted on the following page Yandex smtp settings with ssl
. Hence, I think credits to answer this question should go to them.

Try using port 25 instead of 465 specified in Yandex help. I found this info on https://habrahabr.ru/post/237899/. They mentioned that it might be due to the fact that explicit SSL mode was implemented in the SmtpClient. Then port 25 is used for establishing connection in unencrypted mode and after that, protected mode is switched on.

I had the same problem.
I solved it by going to the Yandex mail, and then change some settings.
Go to:
1- Settings.
2- Email clients.
3- Set selected POP3 setting that is all.