Q: 5.7.57 Client not authenticated to send mail I am a corporate employee.
As part of my work I need to send emails from my C# code.
With the below code I am able to send emails successfully using less secure gmail ID.  But I am not able to do same with my corporate outlook exchange based email ID.
I get below exception.

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox. Visit https://aka.ms/smtp_auth_disabled for more information. [BN9PR03CA0659.namprd03.prod.outlook.com]"

what I should do ?  is there a way I can make my corporate outlook exchange email ID less secure to enable to sending emails.  (Like I did for gmail.)
below is the code I am using.
        MailMessage message = new MailMessage(from, to);
        string mailbody = "In this article you will learn how to send a email using Asp.Net & C#";
        message.Subject = "Sending Email Using Asp.Net & C#";
        message.Body = mailbody;
        message.BodyEncoding = Encoding.UTF8;
        message.IsBodyHtml = true;         
        client.EnableSsl = true;
        client.UseDefaultCredentials = false;
        client.Credentials = new NetworkCredential("name@corporatecompany.com", "$password");            
        client.Host = "smtp.office365.com";
        client.Port = 587;
        client.DeliveryMethod = SmtpDeliveryMethod.Network;          
        try
        {
            client.Send(message);
        }


A: https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission
Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes

*

*Open the Microsoft 365 admin center and go to Users > Active users.

*Select the user, and in the flyout that appears, click Mail.

*In the Email apps section, click Manage email apps.

*Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.

*When you're finished, click Save changes.


A: You need to make sure the smtp address is correct. It may be different for your org, although using O365 and may be port#. Check with your corporate Email managing team/AD team whoever is responsible for this and try again.
It may be that the smtp host can be your org domain host instead of generic one. I am suggesting based on experience of what we have, as we also use O365 for our corporate emails and I have our smtp host as "mail..com" with port#. And there could be a service account which has given appropriate permissions to use as otherwise, it would become less secure to use the smtp relay.
