Troubleshooting mail flow problems in Exchange Server can be a complicated process. There are many factors that can contribute to a slow down in processing and may include messages that are backing up in remote delivery queues due to bad DNS configuration or unintentional third party software settings. As previously discussed in another blog post some messages might be getting backed up in the Messages awaiting directory lookup queue in which case those backed up messages might be due to heavy distribution group expansions or permissions inheritance blocks. It is also possible that messages cannot be received due to a metadata corruption issue.

Some of the ways that the slow down of mail processing is exhibited include:

• Client response is very slow when email status is changed such as when end users change an email’s status from read to unread, or if email items are opened of deleted.
• Messaging Application Programming Interface (MAPI) error messages are generated that include the phrase “Client Operation Failed.”
• Log files in the Mdbdata folder show continuous growth even though very little has actually changed in the public folder resources or the mailbox resources.
• Attempts to improve the speed of processing are unsuccessful such as creating a new folder and moving the contents of the folder, with the slow responsiveness, to the new folder. The move is only temporarily successful as the speed of processing will continue to steadily decrease until the processing matches the previous unacceptable condition.
• Administrators may notice that some folders are more easily accessible than others within the same database and show no slowdown or problems in responsiveness. This uneven distribution of problems and responsiveness can include special folders such as gateway folders like the Mts-in and Mts-out folders.

If users are receiving unexpected non-delivery reports when sending messages then an administrator should review those NDRs to obtain the DSN code that is contained in the non-delivery report. Lookup the DSN code definition and then determine what actions will be needed to correct this error.

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