Sending emails from Azure Communication Service Email using SMTP and application credentials

[biancahuluban]

Hello,
I am trying to send an email by using the scope https://communication.azure.com/.default and SmtpHostUrl = "smtp.azurecomm.net", but I receive at
client.Authenticate(oauth2); the error 535: 5.7.3 Authentication unsuccessful

Basically, I use the following Microsoft documentation and MailKit 4.14.1, but without success(I sent email using the same account with graph scope and Graph nuget, the application id and client secret are well defined, the scope on the application for Graph is SMTP.send email, only that we want to switch to MailKit because we need only to send emails):

https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/send-email-smtp-oauth
Azure application-client secret configuration with these settings:

Did anyone have a successful implementation for sending emails with MailKit with Azure Communication Service SMTP and client credentials? I did not find any example except for the Microsoft one.

[biancahuluban]

I read on this topic about another dev who tried with this scenario of using SMTP application permissions(we have an application client id and client secret with Mail.Send permission)
#1126
saying that " One of the things that I think is unusual is that I've been trying to setup the permissions using application permissions rather than delegated permissions"
Could it be the reason why I receive the client.Authenticate(oauth2) errors using the token that was generated? Why is it required the SMTP UserName and the token? Isn't it the token sufficient?
Or could it be the SMTPUsername that is wrong, I don't know exactly what to set? In some places, I saw the email sender address, in our case if we use the applicationid it says that SMTP username example format is < ACSResourceName >|< ClientID>|< TenantID > , we don t have a SmtpUsername set.

[jstedfast]

Unfortunately, this is not my area of expertise. I am trying to ask around to see if anyone knows the answer to this question.

[natekimball-msft]

@biancahuluban - Please forgive me if these are steps you've already gone through, but I'd suggest taking a look at this doc here: Set up SMTP authentication for sending emails - An Azure Communication Services Article.

I'd also make sure you are adding StartTls to SecureSocketOptions when creating the client. Here's sample usage:

    private static Task SendSmtpMail(MimeMessage message)
    {
        using (var smtpClient = new MailKit.Net.Smtp.SmtpClient())
        {
            smtpClient.Connect(acsConfig.SmtpHostUrl, acsConfig.SmtpPort, SecureSocketOptions.StartTls);
            smtpClient.Authenticate(acsConfig.SmtpAuthUsername, acsConfig.SmtpAuthPassword);

            smtpClient.Send(message);
            smtpClient.Disconnect(true);
        }

        return Task.CompletedTask;
    }