The server response was: 5.7.0 Please authenticate first


I’m trying to switch from SendGrid to Brevo, using the same C# code, just changing my SMTP configuration. I have successfully changed my DNS configuration on CloudFlare and verified it in Brevo. Using the following code with the SMTP settings provided by Brevo, I get the server error « 5.7.0 Please authenticate first »:

    public Task SendEmailAsync(MailMessage mailMessage)
        var client = new SmtpClient(config.Host, config.Port)
            UseDefaultCredentials = false,
            Credentials = new NetworkCredential(config.Username, config.Password, config.Domain)

        return client.SendMailAsync(mailMessage);

Any idea why I’d be getting this error?