ZMailer Overview

[ This is mostly from 1988-1990 by Rayan Zachariassen, some additional notes by Matti Aarnio, 1994-2005 ]

What is this?
This is a package that implements an internet message transfer agent called ZMailer. It is intended for gateways or mail servers or other large site environments that have extreme demands on the abilities of the mailer. It was motivated by the problems of the Sendmail design in such situations.

ZMailer is intended and designed as a multi-protocol mailer. The only protocol supported in this distribution is RFC822 (and variations).

In the current era of SPAM plagues, the ZMailer has also fairly good mechanisms for SMTP input time analysis of the messages; both realtime DNS lookups, and static table analysis methods exists.

What good will it do me?
Different people and sites differ in priorities. Keep that in mind when reading this ``features'' list:

Code and Design features:

Default configuration file features:

What experience shows that it can do?
Original developement systems had loads of 1000-2000 messages a day (somewhat more route decissions, like 2-3 times that), which by todays (-96) standard is nothing to speak about, but multi-router mode has enabled serious processing to happen which sends out 20-50 THOUSAND messages a day per process, and total number becoming in range of millions..

On one burst-load test the system did show up to handle about 12500 messages a day per router process (three route lookups on each message from DNS across the Ethernet). If system can take more route processes, it is most likely possible to increase system performance to hundreds of thousands messages per day.. There is no conclusive evidence that next possible bottle-neck, scheduler, won't clog, but there appears to be ample power (if your machine has it..) -- 50 000 messages a day is no problem. ( Sun SS-10/41 ) [ Apr-1994 ]

Another burst-load test routed 1000 messages to "nobody" (via local alias db to "/dev/null") on an Sun SS-10/50 MHz/Solaris 2.4, gave speed of 120 000 messages per day per router process (the test was run with 4 parallel routers)

On same machine with scheduler from zmailer-2.99.15, scheduling the messages took a bit under two minutes (1:55), which indicates speed of roughly 750 000 messages per day. Very likely it can exceed million messages per day (not million recipients, like when expanding lists, but million individual messages!) [ Aug-1995 ]

With rewritten scheduler (zmailer-2.99.19: scheduler-new) the speed improved still: The speed is actually dominated by the speed the scheduler can assimilate information from new entries.

The ZMailer is used on various multi-domain machines, including ones running Internet, ( and BITNET, and/or UUCP ).

How does it work?
ZMailer is a multi-process mailer, using two daemon processes to manipulate messages. One of these processes is a router (which may consist of N parallel processes), and makes all decisions about what should happen to a message. The other daemon is a message queue manager, used to schedule delivery of messages.

The Router uses a configuration file that closely follows Bourne- shell script syntax and semantics with minimal magic. Message files are moved around in a series of directories, and the Scheduler and its Transport Agents run off of control files created by the Router.

The Router will process messages one at a time, as it finds them in a directory where User Agents submit their outgoing messages. Envelope and Message Header information is all kept in the same message file along with the message body, and this file is never modified by any ZMailer program. After parsing the envelope and RFC822 header information, the Router validates the information extracted, and calls functions defined in the configuration file to decide exactly how to deliver the message and how to transform the embedded addresses. The algorithms that do this are easily reconfigurable, since the control flow and address manipulation is specified by familiar shell script statements. When the Router is finished, it will produce a message control file for use by the delivery processing stage of ZMailer, and move the original message file to another location.

Once the Router has decided what to do with each of the addresses in a message, the Scheduler daemon builds an internal summary of this information by reading the control file created by the Router. This knowledge is merged with a data structure it maintains that stores which messages are supposed to be sent where, and how. According to a pre-arranged agenda, the Scheduler will execute delivery programs to properly move the message envelope, header, and body, to the immediate destination. These delivery programs are called Transport Agents, and communicate with the Scheduler using a simple protocol that tells them which messages to process and returns status reports to the Scheduler. The Scheduler also manages status reports, taking appropriate action on delivery errors, and when all delivery instructions for a message have been processed, deletes remnants of the message from the spool.

There are several standard Transport Agents included with the ZMailer distribution. The collection currently includes a local delivery program, an SMTP client implementation, and a Transport Agent that can run Sendmail-compatible delivery programs.

A separate utility allows querying the Scheduler for the state of its mail queues. For existing Sendmail installations, a Sendmail replacement program is included that simulates most of the Sendmail functionality in the ZMailer environment. This allows ZMailer to replace a working Sendmail installation without requiring changes in standard User Agents.

If you have a client/server host environment, mail clients need not run any daemons. They only need two programs (the Sendmail replacement, and a mail queue querying program) and a view of the mail server's spool area.

How will it affect my current system?
Two programs will be replaced: /usr/lib/sendmail (with a functionality emulator), and /bin/rmail (for completeness). Nothing else will be changed, so it is very easy to back out if you want to.

What problems can I expect?
This software is in use on several major mail gateways. The development machine is a departmental system that processes 2000 messages a day. This mailer has been running on it in some form for 5 years. This means that you are unlikely to see crippling problems in the software. [940829: Rayan was optimist.. Or, well, Serious Hacking to get new MIME-features has caused much bugs to surface.. /mea]

You may well encounter problems due to your local Operating System, libraries, compiler (this software makes a good compiler test suite), or other things about your environment that I have not considered.

The ZMailer Router RFC822 implementation is rather picky about adherence to the protocol standards. You may run into cases where an error that has been ignorable or unknown so far, will generate automatic complaints from the mailer to users, and from users to you. Much of this pickyness is required. Some is not, and is deselectable.

Due to the security mechanism, in particular the propagation and use of privilege levels associated with addresses, the default setup requires that the .forward and mailing list files be publically readable. Otherwise, unprivileged addresses (e.g., those specified from outside the host), will not be able to use the contents of such files. Similarly, if the source of an address is not ``secure'', the address will have no privileges. In particular, for an address to take on the privileges of whomever specified it (as in a .forward file), its source file must be owner-only writable, and in a directory which has the same ownership or is owned by root and is also owner-only writable. These checks may be disabled.

What level of support can I expect?
I will promptly fix problems that I consider serious if they are reproducible on a system I have easy access to. Any comments or reports you send in will be saved and considered. Because of my other activities, I cannot commit to answering non-critical items in a timely manner, or at all. [940829: "I" has latter turned out to be <> ]

... and if you want to pay for support, ask <>

For info about the ZMailer list subscribing and archive, see tech-list info page.

How do I install this?
If you are an SMTP (and possibly UUCP) site running a supported OS, you can use a supplied host environment description and the default Router configuration files.

Read the Install section at the ZMailer manual.

Read the file doc/guides/aliases for information on local address interpretation, and doc/guides/lists for a quick introduction to setting up mailing lists.

© Rayan Zachariassen, 1988-1990, © Matti Aarnio, 1994-2005