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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 some emails are not being received by customers. He already did send an email to a test and account. These emails are being delivered to the test email accounts. Why are the emails of the users 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 users of the company 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 spamfilter between the Exchange Servers and the Internet. This 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 being sent. The email that is being sent goes to the Exchange Servers. The Exchange Servers have a send-connector configured. The send-connector is configured to the on-premises spamfilter.

It looks like the following:

E-mail is sent –> Exchange Servers –> Spamfilter –> Internet.

Log in the on-premises spamfilter. 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 spamfilter 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 will get delivered. After 1 hour, I checked the spam filter and I saw that the queue is getting less than 1000 emails. 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 effecting Office 365 customers.


In this article, you learned why the users are getting an error code 451 4.7.500 error Server busy Non-Delivery Receipt (NDR). If you like to read more about it, Microsoft did write an article about it. 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.

I hope you enjoyed this article. You may also like Remove Exchange certificate with PowerShell. Don’t forget to follow us and share this article.



ALI TAJRAN is a passionate IT Architect and IT Consultant. His specialism is designing and building complex enterprise environments. He started Information Technology at a very young age, and his goal is to teach and inspire others. Connect with ALI TAJRAN on social media. Read more »

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