MailKit.Security.AuthenticationException: '535: 5.7.139 Authentication unsuccessful, the user credentials were incorrect.

Question

I am getting this error "MailKit.Security.AuthenticationException: '535: 5.7.139 Authentication unsuccessful, the user credentials were incorrect." for my mail integration in my ASP.Net Core API Application.

I am using a Microsoft account to send the email, i tested it with PowerShell command and it is working but when i integrate it in my application it is not working.

I have disabled the security defaults and the account does not have any MFA and is excluded in conditional access policy for MFA

These are the settings i have set

"EmailSettings": {

"Host": "smtp.office365.com",

"Port": 587,

"Email": "Email ID",

"Password": "App Password",

"DisplayName": "Display Name",

"EnableSSL": "true"

}

And calling them like this in my code

await _emailService.SendAsync(dto.Email, "Subject", emailbody);

Email Service file:

public async Task SendAsync(string email, string subject, string body)

{

 await SendEmailAsync(email, subject, body);

}

public async Task SendEmailAsync(string toEmail, string subject, string body)

{

var smtpHost = _config["EmailSettings:Host"];

var smtpPortString = _config["EmailSettings:Port"];

var smtpEmail = _config["EmailSettings:Email"];

var smtpPassword = _config["EmailSettings:Password"];

var displayName = _config["EmailSettings:DisplayName"];

var enableSSL = _config["EmailSettings:EnableSSL"];

if (string.IsNullOrEmpty(smtpHost) || string.IsNullOrEmpty(smtpPortString) ||

    string.IsNullOrEmpty(smtpEmail) || string.IsNullOrEmpty(smtpPassword))

{

    throw new ArgumentNullException("Missing required email configuration values.");

}

if (!int.TryParse(smtpPortString, out int smtpPort))

{

    throw new ArgumentException("Invalid SMTP port number.");

}

bool useSSL = bool.TryParse(enableSSL, out var parsedSsl) && parsedSsl;

var secureSocketOption = useSSL ? SecureSocketOptions.StartTls : SecureSocketOptions.Auto;

var message = new MimeMessage();

message.From.Add(new MailboxAddress(displayName, smtpEmail));

message.To.Add(MailboxAddress.Parse(toEmail));

message.Subject = subject;

message.Body = new TextPart("html")

{

    Text = body

};

try

{

    using var client = new SmtpClient();

    Console.WriteLine($"Connecting to SMTP server {smtpHost}:{smtpPort} with SSL: {useSSL}...");

    await client.ConnectAsync(smtpHost, smtpPort, secureSocketOption);

    await client.AuthenticateAsync(smtpEmail, smtpPassword);

    await client.SendAsync(message);

    await client.DisconnectAsync(true);

    Console.WriteLine("Email sent successfully.");

}

catch (MailKit.Security.AuthenticationException authEx)

{

    Console.WriteLine($"Authentication error: {authEx.Message}");

    throw;

}

catch (Exception ex)

{

    Console.WriteLine($"General error: {ex.Message}");

    throw;

}

}

I am getting the error on this line : "await client.SendAsync(message);"

Answer 1

Office 365 no longer supports basic security (username and password). You will need to use oauth2. See this thread:

https://github.com/jstedfast/MailKit/discussions/1768