Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, the request did not meet the criteria to be authenticated successfully. Contact your administrator.

Question

MY code
 string result = string.Empty;

                 var httpRequest = HttpContext.Current.Request;

                MailMessage mail = new MailMessage();

                SmtpClient SmtpServer = new SmtpClient("smtp.office365.com");

                mail.From = new MailAddress("******@amazeinc.in");

                mail.To.Add(EmailEntity.emailto);

                mail.Subject = EmailEntity.emailsubject;

                mail.Body = EmailEntity.emailbody;
 mail.IsBodyHtml = true;

                SmtpServer.Port = 587;

                SmtpServer.UseDefaultCredentials = false;

                SmtpServer.Credentials = new System.Net.NetworkCredential("******@amazeinc.in", "XXXXXXXXXXXXXXXX");

                SmtpServer.EnableSsl = true;

                SmtpServer.Send(mail);

Answer 1

Hi,

I'm getting the same issue, it was about TLS versions, I solved it replacing :

                SmtpClient SmtpServer = new SmtpClient("smtp.office365.com");

with

                SmtpClient SmtpServer = new SmtpClient("smtp-legacy.office365.com");

Can you try it and let us know ?

FYI, I got that trick from :

https://learn.microsoft.com/en-us/answers/questions/1164157/smtp-auth-modern-authentication-in-managed-devices

Answer 2

I am using Direct Send option as described in the Mail Flow best practicesand it works like a charm.

Answer 3

I'm getting the same error in a non-Azure / Active Directory Web Server ASP Net App, that was working at this time last year.

"As suggested in another post"? what good is that? Bing sure isnt going to find it for me.

I really am tired of MS answers that presume platform, or prior knowledge.

Answer 4

Hello Amaze E,

I’m Segunfunmi, an Independent Advisor and Microsoft user like you. Thanks for posting the query here in this forum.

The error message 535 5.7.139 Authentication unsuccessful indicates that the request did not meet the criteria to be authenticated successfully. This issue has several possible causes, including Azure Security Defaults and Conditional Access policy blocking Legacy Authentication1. You may want to check these settings to ensure they are not blocking your authentication attempts.

Another possible solution is to use an App Password, which can be created by enabling/enforcing MFA for the user1. You can also try disabling MFA for the user or using “Authenticate an IMAP, POP or SMTP connection using OAuth,” as suggested in another post.

If you have tried all of these solutions and are still experiencing issues, you may want to check the sign-in logs for the user in Azure Active Directory for more details. I hope this helps! Let me know if you have any further questions.

Kind regards, Segunfunmi.