Ошибка unable to relay recipient in non accepted domain

In case you trying to sent e-mails outside your company using anonymous authentication, you have to run the below commands from Exchange Management Shell.

• Find your receive connector you want to use (ex. Default Frontend EXCHANGESERVER) and execute below commands:

Set-ReceiveConnector "EXCHANGESERVERDefault Frontend EXCHANGESERVER" -PermissionGroups AnonymousUsers

Get-ReceiveConnector "EXCHANGESERVERDefault Frontend EXCHANGESERVER" | Add-ADPermission -User 'NT AUTHORITYAnonymous Logon' -ExtendedRights MS-Exch-SMTP-Accept-Any-Recipient

Done!

При отправке писем с помощью данного метода, письмо отправляются только на доменные п/я. При попытке отправить на иные (Google, Mail.ru) выдаёт Exception

System.Security.Authentication.AuthenticationException: «Удаленный сертификат недействителен согласно результатам проверки подлинности.»

При подобных исключениях, советовали на callback сервера, передавать true (соглашаясь, что сертификат может быть не действительным).
При использовании этого кода (закоменчены несколько вариантов), возвращается

System.Net.Mail.SmtpFailedRecipientsException: «Не удалось отправить всем получателям.»

SmtpFailedRecipientException: Почтовый ящик недоступен. Отклик сервера: 5.7.54 SMTP; Unable to relay recipient in non-accepted domain

Если этот де код использовать для отправки с mail.ru или google, исключений не возникает.

Просьба подсказать, где имеются ошибки или иные варианты решения.
Заранее спасибо.

<pre>
using System;
using System.Globalization;
using System.Net;
using System.Net.Mail;
using System.Net.Security;
using System.Windows.Forms;
using Attachment = System.Net.Mail.Attachment;
using System.Threading;
using System.Security.Cryptography.X509Certificates;

namespace TestProg.DocSearchByParams.SystemMethods
{
    internal class MailSenderClass
    {
        private static void MailTo()
        {
            Thread.CurrentThread.CurrentCulture = new CultureInfo("ru-RU");
            var fromMailAddress = new MailAddress("mail@from.ru", "test");
            var adr = new string[] {"mail11@mail.ru", "mail22@gmail.com" };

            using (var smtp = new SmtpClient())
            {
                var message = new MailMessage
                {
                    From = fromMailAddress
                };

                foreach (var P in adr)
                {
                    message.To.Add(new MailAddress(P));
                }

                message.Subject = "Тема + неделя: "
                                  + DateTime.Now.AddDays(-7) + " - "
                                  + DateTime.Now;
                message.Body = "Тело";
                var attach = @"F:VS2017Документ от " + DateTime.Now.ToShortDateString() + ".xlsx";
                //message.Attachments.Add(new Attachment(attach));

                smtp.Host = "mail.esphere.ru";
                smtp.Port = 25;
                smtp.EnableSsl = true;
                smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
                smtp.Timeout = 30000;
                smtp.UseDefaultCredentials = false;
                smtp.Credentials = new NetworkCredential(fromMailAddress.Address, "Password");


                /* Попытки избавиться от Exception "Сертификат не действителен" 
                 * ServicePointManager.ServerCertificateValidationCallback =
                 * delegate (object sender, X509Certificate certificate, X509Chain chain,
                 * SslPolicyErrors sslPolicyErrors)
                 *  { return true; };
                */

                /*
                 * System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(AcceptAllCertifications);
                 *ServicePointManager.ServerCertificateValidationCallback =
                 *    delegate (object s, X509Certificate certificate,
                 *        X509Chain chain, SslPolicyErrors sslPolicyErrors)
                 *    { return true; };
                 */
                //ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(ValidateServerCertificate);

                smtp.Send(message);

                message.Dispose();
                smtp.Dispose();
            }
        }
      </pre>

1. Authentication is using 587 port, Anonymous connections is using 25 port, please make sure Telnet the port correctly.

By the way, for security reasons, we do not recommend modifying default connectors, I would suggest you create an anonymous relay connector to relay mails from this application.

https://docs.microsoft.com/en-us/exchange/mail-flow/connectors/allow-anonymous-relay Opens a new window

2. 

Very likely the SMTP provider (in your example company.com) has SPF setup. Which is a good and beautiful thing.

What is SPF? «Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain’s administrators»

You might need to contact the Administrator to allow the domain to use SMTP to send email.