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Common Exchange mail delivery issues

This article discusses some common culprits of Exchange message delivery delays and failures.

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Normally, when a user sends an e-mail via Outlook, the message is delivered within just a few seconds. Sometimes, however, unsavory Exchange server conditions arise that create message delivery delays or failures. In this article, I discuss some common culprits at the root of these problems.

If you find yourself troubleshooting these issues, the first thing I recommend checking is your server's Microsoft Exchange MTA Stacks service. Make sure it's running -- if it is running, try restarting it. This isn't a magical cure-all method, but I have occasionally been able to unstick a queue by restarting this service.

If your Exchange server is still having trouble sending mail outside the company, the first thing you should check for is a link failure. Make sure that the malfunctioning server can actually see the Internet, and that any necessary routers, bridgehead servers, gateway servers, etc., are up and running. Of course, if users are able to receive mail from the outside world, this in itself proves that your communications links are working.

If you've confirmed that your server can communicate externally, but it's still not sending Internet mail, the problem could be related to server configuration (especially if it's a new server). If Exchange's SMTP virtual server is not configured with the correct DNS information, outbound messages destined for the Internet may back up in the queue and never be transmitted.

To determine whether or not a DNS configuration issue is your problem:

  1. Open Exchange System Manager and navigate to Administrative Groups -> your administrative group -> Servers -> your server -> Protocols -> SMTP -> Default SMTP Virtual Server (or the SMTP virtual server that you are having problems with).

  2. Right click on the Default SMTP Virtual Server and select Properties.

  3. Select the Delivery tab and then click the Advanced button.

  4. Make sure that the server's fully qualified domain name is entered into the Fully Qualified Domain Name field. You can verify that the name entered is valid by clicking the 'Check DNS' button.

  5. Now, click the Configure button and you will see a list of the external DNS servers used by the SMTP virtual server.

According to Microsoft (KB 326992), if you use an Internet service provider to connect your network to the Internet, and your server isn't configured with the ISP's DNS server address, the server won't be able to send e-mail to Internet users. You can correct the problem by adding the ISP's DNS server address to the list of external DNS servers.

Filling in a list of external DNS servers isn't an absolute requirement though. You can also solve the problem by adding a forwarder to the DNS server used by your Active Directory. A forwarder is basically an entry that tells the DNS server to forward requests to your ISP's DNS server if it is unable to resolve the request itself.

So what if messages are being delivered, but the server is just taking forever to deliver them? Well, again, there are several things that can cause this problem, but the issue more often than not relates to hardware.

If an Exchange server has insufficient memory, hard disk space, disk throughput, or network bandwidth, message delivery can slow to a crawl. Often, if the queues are backing up and message delivery is slow, it's because the hard disk simply can't keep pace with the demand.

The easiest way to fix the problem is to install a faster drive (preferably a disk array) and move the queues to it. You can control the queue locations through the following registry key:


The key name is Working Directory. (You should perform a full system backup before making any registry modification.)

About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. Brien has served as the CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at


We found that the Microsoft firewall can be the culprit of slow mail delivery at times. We had some users having slow mail problems, but not others. The problem went away when we made Outlook an exception in the firewall program.
—Dave F.

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Related information from

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  • Reference Center: Tips and expert advice on Exchange and DNS configuration and troubleshooting

  • Dig Deeper on Exchange Server setup and troubleshooting

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    i am able to send the email and end user has been received email, but when end user send the email to us that time we ll receive email very late like 30 min to 5 hours.