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sendmail Cookbook by Craig Hunt

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Chapter 9. Managing the Queue

Introduction

The queue is where mail is held until it can be successfully delivered. The queue holds mail for later delivery when a temporary error is encountered during the delivery process. Most sendmail administrators have seen “Deferred” appear in an error message and know that it means the mail will be held in the queue until the next queue run. Mail is also placed in the queue when the system load exceeds the load average defined by the sendmail.cf QueueLA option, and when certain delivery modes are used. Queuing is an important component of creating a robust, reliable mail delivery system.

Each mail message in the queue can be represented by up to four queue files:

  • The df file contains the message body.

  • The qf file contains the message headers and the queue control information needed to deliver the message.

  • The xf file is a temporary file that contains a transcript of any error messages sendmail receives when attempting to deliver the message. Any error messages collected in the xf file are returned to the sender, and the xf file is deleted after the delivery attempt.

  • The tf file is a temporary work file that sendmail uses to produce the updated qf file. The queue control information in the qf file is updated each time a message is requeued. Any changes to that information take place in the tf file to safeguard the qf file. After the tf file is successfully updated, it is renamed as a qf file.

Of the four file types, only the df and qf files ...

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