got exception authenticate failed when AuthenticateAsync()

[karnse]

I try to connect IMAP server with this code
code

private async Task<string> GetAccessTokenAsync(CancellationToken stoppingToken)
{
    var app = ConfidentialClientApplicationBuilder
            .Create(_clientId)
            .WithAuthority(new Uri($"https://login.microsoftonline.com/{_tenantId}/v2.0"))
            .WithClientSecret(_clientSecret)
            .Build();
    var result = await app.AcquireTokenForClient(new[] { "https://outlook.office365.com/.default" }).ExecuteAsync(stoppingToken);
    return result.AccessToken;
}

protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
    while (!stoppingToken.IsCancellationRequested)
    {
        try
        {
            _client.Connect("outlook.office365.com", 993, SecureSocketOptions.Auto);
            var accessToken = await GetAccessTokenAsync(stoppingToken);
            var auth = new SaslMechanismOAuth2(_email, accessToken);
            _client.AuthenticationMechanisms.Remove("PLAIN");
            await _client.AuthenticateAsync(auth, stoppingToken);
            await CheckForNewMessagesAsync(stoppingToken);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
            if (ex is MailKit.Security.AuthenticationException authEx)
            {
                Console.WriteLine($"Authentication failed: {authEx.InnerException?.Message}");
            }
            await Task.Delay(TimeSpan.FromSeconds(30), stoppingToken);
        }
        finally
        {
            if (_client.IsConnected)
            {
                await _client.DisconnectAsync(true, stoppingToken);
            }
        }
    }
}

server client log

Connected to imaps://outlook.office365.com:993/
S: * OK The Microsoft Exchange IMAP4 service is ready. [SwBVADEAUABSADAAMwBDAEEAMAAwADIANAAuAGEAcABjAHAAcgBkADAAMwAuAHAAcgBvAGQALgBvAHUAdABsAG8AbwBrAC4AYwBvAG0A]
C: A00000000 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=PLAIN AUTH=XOAUTH2 SASL-IR UIDPLUS MOVE ID UNSELECT CHILDREN IDLE NAMESPACE LITERAL+
S: A00000000 OK CAPABILITY completed.
C: A00000001 AUTHENTICATE XOAUTH2 ********
S: A00000001 NO AUTHENTICATE failed.

[jstedfast]

The problem isn't your code. The problem is that your auth token is bad.

I can't help you beyond that. You'll have to contact your support rep at Microsoft to ask for help.