Service not available, closing transmission channel. The server response was: Service not available

Question

I'm encountering an issue while trying to send emails from my .NET application using the SmtpClient class. Even though my credentials are correct, I keep getting the following error:

System.Net.Mail.SmtpException: Service not available, closing transmission channel. The server response was: Service not available

Sometimes, after multiple attempts, I also get a timeout error.

What I've tried:

• Verified that my SMTP server (smtp.gmail.com) is correct and the port is set to 587
• Confirmed that my username and password are accurate
• Enabled SSL based on the configuration settings
• Made sure that less secure apps are allowed in my Gmail account settings

Despite these efforts, the issue persists. Below is the relevant code snippet:

public class EmailService
{
    private readonly IConfiguration _configuration;
    private readonly ILogger<EmailService> _logger;
 
    public EmailService(IConfiguration configuration, ILogger<EmailService> logger)
    {
        _configuration = configuration;
        _logger = logger;
    }
    
    public async Task<EmailResponse> SendEmailAsync(EmailRequest emailRequest)
    {
        using (MailMessage mailMessage = new MailMessage())
        {
            mailMessage.From = new MailAddress(emailRequest.Sender);
            foreach (var recipient in emailRequest.Receipient)
            {
                mailMessage.To.Add(new MailAddress(recipient));
            }
            mailMessage.Subject = emailRequest.Subject;
            mailMessage.Body = emailRequest.Body;
    
            var configurationKeys = _configuration.GetSection("EmailConfig").Get<EmailConfig>();
            if (configurationKeys == null)
            {
                _logger.LogError("No email credentials available.");
                throw new ArgumentNullException(nameof(configurationKeys), "No email credentials                    available.");
            }
    
            var smtpClient = new SmtpClient("smtp.gmail.com")
            {
                Port = 587,
                UseDefaultCredentials = false,
                Credentials = new NetworkCredential(configurationKeys.Username, configurationKeys.Password),
                EnableSsl = configurationKeys.EnableSSL?.ToLower() == "true",
            };
    
            try
            {
                await smtpClient.SendMailAsync(mailMessage);
                _logger.LogInformation("Email sent successfully.");
                return new EmailResponse { Status = "successful", Message = "Email sent                successfully." };
            }
            catch (SmtpException smtpEx)
            {
                _logger.LogError(smtpEx, "SMTP error occurred while sending email.");
                return new EmailResponse { Status = "failed", Message = smtpEx.Message };
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "An error occurred while sending email.");
                return new EmailResponse { Status = "failed", Message = ex.Message };
            }
        }
    }
}

Answer 1

Yep, the issue is not from the code, but has to do with security, because my endpoint is http when I send the email, it fails when deployed and make the request where it has a valid https endpoint it goes through successfully.

Answer 2

You could try this below code:

using System;
using System.Net;
using System.Net.Mail;
using System.Threading.Tasks;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.DependencyInjection;

namespace EmailSenderProject
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            // Set up configuration and logging
            var serviceProvider = new ServiceCollection()
                .AddLogging(configure => configure.AddConsole())
                .AddSingleton<IConfiguration>(new ConfigurationBuilder()
                    .SetBasePath(AppContext.BaseDirectory)
                    .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                    .Build())
                .BuildServiceProvider();

            var logger = serviceProvider.GetService<ILogger<Program>>();
            var configuration = serviceProvider.GetService<IConfiguration>();

            var emailService = new EmailService(configuration, logger);

            var emailRequest = new EmailRequest
            {
                Sender = "sender address",
                Receipient = new[] { "receiver address" },
                Subject = "Test Email",
                Body = "This is a test email sent from a .NET application."
            };

            var response = await emailService.SendEmailAsync(emailRequest);
            Console.WriteLine(response.Message);
        }
    }

    public class EmailService
    {
        private readonly IConfiguration _configuration;
        private readonly ILogger<Program> _logger;

        public EmailService(IConfiguration configuration, ILogger<Program> logger)
        {
            _configuration = configuration;
            _logger = logger;
        }

        public async Task<EmailResponse> SendEmailAsync(EmailRequest emailRequest)
        {
            using (var mailMessage = new MailMessage())
            {
                mailMessage.From = new MailAddress(emailRequest.Sender);
                foreach (var recipient in emailRequest.Receipient)
                {
                    mailMessage.To.Add(new MailAddress(recipient));
                }
                mailMessage.Subject = emailRequest.Subject;
                mailMessage.Body = emailRequest.Body;

                var configurationKeys = _configuration.GetSection("EmailConfig").Get<EmailConfig>();
                if (configurationKeys == null)
                {
                    _logger.LogError("No email credentials available.");
                    throw new ArgumentNullException(nameof(configurationKeys), "No email credentials available.");
                }

                using (var smtpClient = new SmtpClient("smtp.gmail.com"))
                {
                    smtpClient.Port = 587; // Make sure the port is 587 for TLS
                    smtpClient.UseDefaultCredentials = false; // This should be false since you are providing credentials
                    smtpClient.Credentials = new NetworkCredential(configurationKeys.Username, configurationKeys.Password);
                    smtpClient.EnableSsl = true; // Ensure SSL is enabled

                    try
                    {
                        await smtpClient.SendMailAsync(mailMessage);
                        _logger.LogInformation("Email sent successfully.");
                        return new EmailResponse { Status = "successful", Message = "Email sent successfully." };
                    }
                    catch (SmtpException smtpEx)
                    {
                        _logger.LogError(smtpEx, "SMTP error occurred while sending email.");
                        return new EmailResponse { Status = "failed", Message = smtpEx.Message };
                    }
                    catch (Exception ex)
                    {
                        _logger.LogError(ex, "An error occurred while sending email.");
                        return new EmailResponse { Status = "failed", Message = ex.Message };
                    }
                }
            }
        }
    }


    public class EmailRequest
    {
        public string Sender { get; set; }
        public string[] Receipient { get; set; }
        public string Subject { get; set; }
        public string Body { get; set; }
    }

    public class EmailResponse
    {
        public string Status { get; set; }
        public string Message { get; set; }
    }

    public class EmailConfig
    {
        public string Username { get; set; }
        public string Password { get; set; }
        public string EnableSSL { get; set; }
    }
}

And make sure you have enabled 2FA on google account you are using and set up the app password.

https://support.google.com/accounts/answer/185833

appsettings.json:

{
  "EmailConfig": {
    "Username": "sender email",
    "Password": "app password",
    "EnableSSL": "true"
  }
}