Anatomy of Postfix

Developed with security and speed in mind, Postfix has become a popular alternative to Sendmail. The Book of Postfix published by No Starch Press is a complete guide to Postfix whether used by the home user, as a mailrelay or virus scanning gateway, or as a company mailserver. Practical examples show how to deal with daily challenges like protecting mail users from SPAM and viruses, managing multi
Postfix Queues

Postfix polls all queues in the directory specified by the queue_directory parameter in your file. The queue directory is usually /var/spool/postfix. Each queue has its own subdirectory with a name identifying the queue. All messages that Postfix handles stay in these directories until Postfix delivers them. You can determine the status of a message by its queue: incoming, maildrop, deferred, active, hold, or corrupt.


All new messages entering the Postfix queue system get sent to the incoming queue by the cleanup service. New queue files are created with the postfix user as the owner and an access mode of 0600. As soon as a queue file is ready for further processing, the cleanup service changes the queue file mode to 0700 and notifies the queue manager that new mail has arrived. The queue manager ignores incomplete queue files whose mode is 0600.

The queue manager scans the incoming queue when moving new messages into the active queue and makes sure that the active queue resource limits have not been exceeded. By default, the active queue has a maximum of 20,000 messages.

Caution: Once the active queue message limit is reached, the queue manager stops scanning the incoming and deferred queues.


Messages submitted with the sendmail command that have not been sent to the primary Postfix queues by the pickup service await processing in the maildrop queue. You can add messages to the maildrop queue even when Postfix is not running; Postfix will look at them once it is started.

The single-threaded pickup service scans and drains the maildrop queue periodically, as well as upon notification from the postdrop program. The postdrop program is a setgid helper that allows the unprivileged sendmail program to inject mail into the maildrop queue and notify the pickup service of message arrival. (All messages that enter the main Postfix queues do so via the cleanup service.)


If a message still has recipients for which delivery failed for some transient reason, and the message has been delivered to all the recipients possible, Postfix places the message into the deferred queue.

The queue manager scans the deferred queue periodically to put deferred messages back into the active queue. The scan interval is specified with the queue_run_delay configuration parameter. If the deferred and incoming queue scans happen to take place at the same time, the queue manager alternates between the two queues on a per-message basis.


The active queue is somewhat analogous to an operating system's process run queue. Messages in the active queue are ready to be sent, but are not necessarily in the process of being sent.

The queue manager is a delivery agent scheduler that works to ensure fast and fair delivery of mail to all destinations within designated resource limits.

Note: Although most Postfix administrators think of the active queue as a directory on disk, the real active queue is a set of data structures in the memory of the queue manager process.


The administrator can define smtpd access(5) policies and cleanup header and body checks (see Chapter 10) that cause messages to be automatically diverted from normal processing and placed indefinitely in the hold queue. Messages placed in the hold queue stay there until the administrator intervenes. No periodic delivery attempts are made for messages in the hold queue. You can run the postsuper command to manually put messages on hold or to release messages from the hold queue into the deferred queue.

Messages can potentially stay in the hold queue for a time that exceeds the queue file lifetime set by the maximal_queue_lifetime parameter (after which undelivered messages are bounced to the sender). If older messages need to be released from the hold queue, you can use postsuper -r to move them into the maildrop queue, so that the message gets a new timestamp and is given more than one opportunity to be delivered.

Note: The hold queue doesn't play much of a role in Postfix performance; monitoring of the hold queue is typically motivated by tracking spam and malware rather than by performance issues.


The corrupt directory contains damaged queue files. Rather than discarding these, Postfix stores them here so that the (human) postmaster can inspect them using postcat.

Postfix logs a warning about any corrupt files upon startup.



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Anonymous's picture

the f**k out people. you're not learning anything by having a go at each other.

Postfix is easier to install

V. Prague's picture

You say that Postfix is a useful alternative to Sendmail, but it is actually better.
Just a week ago, I was unable to set up Sendmail properly on the 64 bit Mandrake - it just refused to send mail. I asked a very experienced friend to do this, only to hear that Sentmail apparently does not respect some DNS related settings. He installed Postfix and it works perfectly. I do not wish to hear about Sendmail again.

This is THE Postfix book to have

Scott Kitterman's picture

I found the Book of Postfix invaluable to me when I was getting started with Postfix. If you want to learn Postfix, this book is the place to start.

This is THE Postfix book to have - hardly

Anonymous's picture

Postfix specialists might disagree with you. I certainly do.

2nd edition?

Chris's picture

The book has an extraordinary amount of errata -- so much that I have to keep the errata page open whenever I read the book. Are there plans for a second (proof-read) edition?

A second edition is in the

Anonymous's picture

A second edition is in the works. Unfortunately we need to backport the text from RTF to our native XML format first :(

Why bother?

Anonymous's picture

Basically, you're promoting your book, much of which I have read. The article itself is simply a brain dump for you to jack up your ego and say, "look what I know". Only a postfix administrator or developer would understand this article, so it teaches little - like your book.

Readers might need a warning: No Starch Press did a poor job of editing the book (if they did any editing at all): No copy editing, no technical editing, etc. and most of it reads like a German with little understanding of native English - "now, I will tell you how you must do this....First you must ...then you must, etc."

Additionally, the main Chapter about building a company server is broken. You won't build a functional server following their instructions.

So, consider this with caution.

Considering this is the only

Anonymous's picture

Considering this is the only article online I've found that explains clearly the process that postfix uses to process mail, and how all the pieces fit together, I hardly see it as just an ego braindump.

The entire postfix documentation is written so that only a postfix administrator or developer would understand it.

Considering that every organisation is going to do mail a little differently, standard how-to guides very rarely can be followed completely. This is why it's so important to actually understand the system. If I can understand the system, each configuration value is just a manpage away.

Why bother....

Keith Daniels's picture

Uh.... I assume you have written something better? If so I would like to hear about it...

As far as the promotion goes, the authors had nothing to do with it, if that is who you are accusing of promoting it.

RE: "Only a postfix administrator or developer would understand this book"...... duh, who else but someone who was, or wanted to be, an administrator or developer WOULD read it?

If formal American style English is your prime criteria for the excellence of a book and not having that makes it not worthy of reading -- then your basic orientation must be academic liberal arts instead of technical. Either that or you have a personal issue with the authors or their nationality and are just trying to put them, the book and their nationality down because of prejudice or personal dislike.

RE: the Chapter about building a company server -- did you check their web site for corrections they might have posted for that page? Did you actually try to build that server setup or is that just your opinion from reading it?

All the new OSs and windowing systems are oriented towards content consumption instead of content production.

--Steve Daniels 2013

Yea, yea, yea.

Anonymous's picture

Yea, yea, yea.

If only an experienced postfix admin could read it, why bother?

In other words, if you're interested in learning postfix, how about a book that guides someone with less knowledge to become more able. That seems to be the purpose of a book like this.

As far as American English - how about just English instead of the horrible language used. Try this from page 313: "Once we got this going, we will make the system more complex." then, "You should have profound understand of LDAP schema and OpenLDAP before you start to implement the company mail server we describe in this chapter".

This book is crammed full of this kind of grand work.

Take your sophomoric diatribe to Slashdot, freak.

yea, yea, yea

Keith Daniels's picture

What's the matter? Your little company having trouble using Postfix with their "Controled Email" and you are blaming it on some poor non native English speakers who wrote a book?

All the new OSs and windowing systems are oriented towards content consumption instead of content production.

--Steve Daniels 2013

yea, yea, yea

Anonymous's picture

DANG! I read two other books before this, both of those helped very little and go me nowhere. I had no previous experience other than HOWTO's I found on line, which weren't many. After I read this book, I can honestly say that I had a successful Postfix email server running and operating SPAM FREE.

These cats don't know what they are talking about! It's a good book and Postfix is definitely easier and faster to learn than bloated sendmail.