Any interruption in the Exchange mail flow and email performance can be equally disturbing as a corrupted Exchange database. In both the cases, the users find the difficulty to access or receive emails. When an email message has not reached to its targeted mailbox, then it may require some troubleshooting to repair the mail flow problems. Even though you have productively configured Simple Mail Transfer Protocol (SMTP) for the desired Exchange Server, there are chances that you may still face mail flow issues. With the help of SMTP queues messages, the user can gain detailed insight of the email delivery and mail flow problems.

Understanding SMTP Queues

To efficiently deliver the emails internally as well as externally, the SMTP uses SMTP queues. In the process of email categorization and delivery, all the email passes though SMTP queues because of its powerful queuing engine. However, at any point in the process, if the delivery of email becomes unsuccessful, then then the email will get stuck within the queue where the issue has encountered. The SMTP queues are helpful in isolating the relevant causes of issues in mail flow.

How To Resolve Exchange Mail Flow Issues Using Smtp Queues?

The below mentioned are some of the common issues in mail flow, which can be tackled using SMTP queues. They are provided with descriptions to allow you to get insight of the issues occurring in mail flow and email performance.

1. Problems to Locate Email Message in SMTP queues: Prior to the task of resolving Exchange Server issues using the location of message, it is important to know location of that email message. To do this, the user can implement the below-mentioned process:

Exchange System Manager >> Administrative Groups >> your administrative group >> Servers >> the problematic server >> Queues

When you just choose Queues container, then the Exchange System Manager wizard will present all the contained SMTP queues of the server inside the interface’s details pane.

As shown in the above figure, the user will receive summary information about the number of emails available in the queues. This is a sufficient information giving hint about the problems occurring in SMTP queues.

It becomes simple to evaluate the real-time location of the messages, as shown in the figure (Queue 1out of 10). Another way of detecting the location of the email is to use the Find Message button. The find button includes ‘DSN messages pending submission’, which contains the delivery status notification.

Though, finding the exact email message inside the SMTP queues can be troublesome and tricky (increases with the number of queues), but it is an essential phase in the troubleshooting process of the mail flow.

2. Fixing Issues of Messages Awaiting Directory Lookup: This SMTP queue indicates the problem that the recipient has is not yet repaired in the MS Active Directory. The emails get amassed in this queue because the queuing engine fails to categorize the message that how to route the inbound messages in the server.

Although there are a few things to troubleshoot them:

  • When global catalog server has problem, then try to designate another domain controller.
  • When occurs any link failure between server and global catalog server, or simply global catalog server is gone offline, then try to restore the servers to fix the issues.

3. Handing Queue Issues of Messages Waiting to Be Routed: In this SMTP queue, the message is held till the targeted destination is not determined. Later it is transferred to the associated link queues. If emails get hung up, then it is often because of routing issues. Though, this issue can be little tricky, because it can be due to corrupted routing table, link failure or because of other reasons. Try to perform Traceroute for the destination domain of the message. This method will help you to determine whether the Exchange can communicate or not, and successfully route the packets to the targeted domain.


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