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

You seem to be passing the From address as emailAddress, which is not a proper email address. For Office365 the From needs to be a real address on the Office365 system.

You can validate that if you hardcode your email address as the From and your Office 365 password.

Don't leave it there though of course.

@Reshma- In case you have not figured it yet, here are below things that I tried and it solved the same issue.

1. Make sure that NetworkCredentials you set are correct. For example in my case since it was office SMTP, user id had to be used in the NetworkCredential along with domain name and not actual email id.

2. You need to set "UseDefaultCredentials" to false first and then set Credentials. If you set "UseDefaultCredentials" after that it resets the NetworkCredential to null.

Hope it helps.

I spent way too much time on this and the solution was super simple. I had to use my "MX" as the host and port 25.

        var sClient = new SmtpClient("domain-com.mail.protection.outlook.com");
        var message = new MailMessage();

        sClient.Port = 25;
        sClient.EnableSsl = true;
        sClient.Credentials = new NetworkCredential("user", "password");
        sClient.UseDefaultCredentials = false;

        message.Body = "Test";
        message.From = new MailAddress("[email protected]");
        message.Subject = "Test";
        message.CC.Add(new MailAddress("[email protected]"));

        sClient.Send(message);