Office365 SMTP: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM

Hi Barry,

Thanks for your answer.

Yes, I am using SMTP client submission.

This is the c# code:

                var smtpClient = new SmtpClient()

                {

                    Host = "smtp.office365.com",

                    Port = 587,

                    UseDefaultCredentials = false,

                    EnableSsl = true

                };

                smtpClient.Credentials = new NetworkCredential("******@test.nl", "xxxxx");

                var message = new MailMessage

                {

                    From = new MailAddress("******@test.nl"),

                    Sender = new MailAddress("******@test.nl"),

                    Subject = "Test mail",

                    IsBodyHtml = false

                };

                message.To.Add("******@test.nl");

                message.Body = "This is a test mail. ";

                smtpClient.Send(message);

The problem is that this code is working fine most of the time; we are sending hundreds of mails every day like this without any problem. But every now and then the SMTP server returns this error and when we try to send the same mail again it sometimes succeeds and sometimes fails again.

I don't think the problem is releated to the settings at out side since it is working most of the times.

I'm having this problem.

In my case, I've been using the smtp server to send messages for the past couple of years with no problem.  However, something happened, and since late July, messages haven't been sent.

So I made a dummy ASP page, got 

5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM

I do not believe I have made any changes that would have cause previously-functioning code to stop functioning.

Hi Corne,

For further investigating, could you please test whether this problem also occurs when you only use the Outlook client? We’d like to know if this problem is related to Outlook client or the SMTP server.

Regards,

Barry

The previous user didn't mention anything, but I can say that logging in through office 365 and sending an e-mail through Outlook works just fine.

Hello Barry,

I can confirm that the limit of 30 messages per minute is not the problem. We have constructed our SMTP client in such a way that it will never send more than 26 messages each minute.

Sometimes the problem occurs after a few minutes of inactivity, so the 30 messages limit is not the cause of the problem. I also would expect another error message in that case.

The limit of 10,000 recipients is also not the problem because most of the times the problem occurs once. After a retry, a few seconds later, the delivery succeeds most of the times.

We do not use a fixed sender address but always a sender address in the same domain. When I use a sender address from another domain I get the error: 5.7.60 SMTP; Client does not have permissions to send as this sender. Since my current problem is about another error message I guess the sender address is not the cause.

I hope you have other suggestions.

Hello Corne,

Based on your description, I assume that you are using SMTP client submission to send emails. Am I correct?

How to set up a multifunction device or application to send email using Office 365

If yes, please make sure that port 587 is allowed in your firewall settings and TLS is enabled. To check if the SMTP service is working fine, please set up the account via POP/IMAP in the Outlook client and check if you can successfully send a test email using the Outlook client.

POP and IMAP account settings

Also, please try using another valid Office 365 mail account as the logon credential and check if the problem can be solved.

Finally, please give us a screenshot of the detailed application settings so we can check if they are all correct from our side.

Thanks,

Barry