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Error code 451 4.7.500 Server busy email delivery issues

Are you getting an error code 451 4.7.500 Server busy email delivery issues? A user called and said that customers are not receiving some emails. He already did send an email to a test and account. These emails do get delivered to the test email accounts. Why are the users’ emails not being delivered to the recipients outside the organization? He likes to know what is happening as he can’t seem to figure it out.

Some company users are receiving a bounce-back of the email after sending an email. The email is sent to recipients outside the organization. The company got two Exchange 2013 on-premises Servers. There is an on-premises spam filter between the Exchange Servers and the Internet. It means that we need to troubleshoot and find a solution. In this article, you will find the solution for Error code 451 4.7.500 Server busy email delivery issues.

Troubleshoot error code 451 4.7.500

First, run Exchange Management Shell as administrator. Run the Get-Queue command. The queue was empty. All the items are sent. The email that is sent goes to the Exchange Servers. The Exchange Servers have a send-connector configured. The send-connector is configured to the on-premises spam filter.

It looks like the following:

E-mail is sent –> Exchange Servers –> Spam filter –> Internet.

Sign in to the on-premises spam filter. Find the send report queue and click analyze. It did show more than 1000+ emails in the queue. Check some emails and click on retry. The spam filter will try to resend the emails. It does show a fail message. Let’s have a look at the logs. The message log is showing: 451 4.7.500 Server busy. Please try again later from [*.*.*.*]. (*******) [*******].

Microsoft Exchange Online Protection (EOP) is a cloud-based email filtering service that helps protect your organization against spam and malware. It includes features to safeguard your organization from messaging-policy violations. EOP can simplify the management of your messaging environment and alleviate many of the burdens that come with maintaining on-premises hardware and software.

Solution error code 451 4.7.500

The IP of the company is being throttled; it’s not blacklisted. The issue is with the Microsoft feature Exchange Online Protection (EOP) service within Office 365. The organization does not have Office 365 configured.

We called the Office 365 Team and explained the situation. They only need your public IP. They did understand and said that within a couple of hours, the email would get delivered.

After 1 hour, I checked the spam filter, and I saw that the queue was getting less than 1000 emails. So I did a flush for all the emails. The emails left the spam filter to the recipients.

If you have Office 365

  • Online support – Sign in and create a service request. Write the error code 451 4.7.500 in the request ticket.
  • Phone support – Contact Office 365 Team by phone and tell them about the error code 451 4.7.500.

If you don’t have Office 365

  • Ask one of the customers that are not receiving the email to get online or phone support with Office 365. They can tell them the issue, and the Office 365 Team will handle it from there.
  • Phone support – Contact Office 365 Team by phone and tell them about the 451 4.7.500 error. Even if you don’t have an Office 365 subscription, they will help you as it’s affecting Office 365 customers.

Keep on reading: Whitelist domain in Office 365 to bypass SPAM filtering »


You learned why the users get an error code 451 4.7.500 error Server busy Non-Delivery Receipt (NDR). Call Microsoft, and they are more than happy to help you out. It will take 1 to 2 hours before the email will send to the recipients outside the organization. If you like to read more about it, Microsoft did write an article about it.

Did you enjoy this article? You may also like Test outbound mail flow in Exchange Server. Don’t forget to follow us and share this article.



ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

This Post Has 2 Comments

  1. Thanks for this article, we have this problem at this time and we are going to migrate to office365 soon but our MX Record are always on our legacy postfix server.
    When Microsot created our admin account for Office365 (no Exchange online configuration for the moment) we met this problem.
    Do you think we need to create connectors with IP of our actual IP on Exchange Online or do we have to create a support ticket to unblock us ?

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