This is one of several pages on WikiLearn that tries to explain email.

If the sketch on this page makes sense to you, along with the variations and notes shown on EmailServerSketches, you understand a lot about how email is handled in Linux (and on the Internet). (Not everything, but a lot.) If it doesn't make sense to you immediately, but you can make sense of it, you will have learned a lot.

This is one of several pages on WikiLearn that tries to explain email. Some of the others are listed below. Eventually they may be combined, but for now they are not. It may be worthwhile looking at some of these pages for a possibly different perspective. In particular, where this page includes small sketches showing various parts of an email server, the EmailSketchWorksheet shows a comprehensive sketch of email handling in a local email server. Also, it has (will have) notes to describe the email configuration I set up on my home LAN.

• EmailServerSketches
• EmailOnLinux (a Table of Contents page, with links to many other pages relevant to email)

See:

• AboutThesePages
• EditingATWikiDrawSketch

Introduction to this Sketch

Eventually I might create text that explains the drawing to help with your understanding. Initially I think it's important to list some of the things that aren't shown on the sketch (so you have some ideas about what the sketch doesn't show):

• The use of procmail for automatically sorting the mail in a user's inbox to his mail folders (or trash, or other users) based on various criteria. This can help to reduce spam.
• The use of virus "filters" like spamassassin, tmda, etc., sometimes to delete email with Windows virii on a Linux email server.
• Some other useful "tricks" that might be encouraged, like ISPs providing user configurable filters (e.g., procmail) on their servers so that unwanted email is not even downloaded. (Someday, maybe this filtering would migrate further upstream, so that unwanted email could be filtered very close to its source.)
• The use of other email and transfer protocols (like UUCP)
• The use of the IMAP protocol for a (windows style) mail client
• The many possible variations in a server, like:
  • Using different MTAs (sendmail, qmail), MDAs (postfix as MDA, ??)
  • Using getmail instead of fetchmail
  • Using a configuration like fetchmail plus procmail to handle incoming mail, in conjunction with options like sending mail from clients on the server or on the LAN directly back to the ISP using smtp to port 25 on the ISP's mail server
• The distinction between header addresses and envelope addresses, and the implications thereof (see EmailEnvelopeAddress).

Some of these variations (or significant portions of them) may be shown on other sketches, but some will almost surely not be shown (unless someone else adds sketches).

Printing this Page

The following is probably no longer correct. I would like to add a heading to contain only the drawing and then use the "page break after" feature HTML 4 to make sure the sketch starts at the top of a new page if printed. (Of course, that would only work in browsers that comply with (that portion) of the HTML 4 standard -- I don't know whether any do or not. Also, I'll have to Google for the instructions to do so.) (Also, at some point I want to move this note to a page like AboutThesePages as a "policy" for WikiLearn.)

• A fortuity is that if I print the sketch, the most important parts (from the top down to and including the large "Figure 1: ..." caption, and to the right to include half of the Internet block labeled "and so on") fit on the first 8 1/2 by 11 sheet of paper. I'd like to keep it that way, and maybe even reduce the width a little bit more. If someone else prints it and gets different results, I'd like to hear about it. (I print from IE5 to a Canon BJC-3000 printer. My screen resolution (which I think is irrelevant) is 800x600. Another thing irrelevant to the sketch, but relevant to how WikiLearn looks to me is that I use a fixed font in IE5 instead of a variable font (I should consider adding this last note to AboutThesePages).

The Email Overview Sketch

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Miscelaneous Notes on Email (to be moved elsewhere)

• Fetchmail can use SMTP or some variety of it -- on the fetchmail (1) man page read about ETRN and ODMR (I found this around the time I wrote back to J. Craig Woods, but I don't think I mentioned it in the response to him).
• For a block diagram of the "internals" of postfix, see http://www.postfix.org/big-picture.html
• quoted from J Craig Woods, about the interaction of postfix and procmail:

2) The MTA, for example Postfix, takes the incoming or outgoing mail, and puts it into an active que. Procmail will now come into play. If the MTA determines that the outgoing email is not for the private network, it will hand it off to the appropriate internet mail server, and this determination is made via dns. If the MTA determines the mail is for some internal machine on the network, it will execute the procmail system, and procmail will deliver it to "/var/spool/mail/user" That is why you setup the command "/usr/bin/procmail - $DOMAIN -d $LOGNAME" in the MTA config file (main.cf).

3)So incoming mail, brought into the network by the MTA, is put into an active queue, and then executes promail for delivery to the actual user.

Commments on This Page (and ToDos)

I wanted and got some good comments on the technical aspects of the drawing from J. Craig Woods and have made some edits in response to them. Some of the comments may be incorporated in existing (or new) drawings on EmailServerSketches.

I've also tried to improve the drawing aesthetically by doing things like rearranging the blocks to reduce the number of crossed lines, making things more consistent (use of colors, terminology, block size), reducing the overall width of the drawing, providing colored backgrounds to better group funtionality (by Internet, Email Server, and Local Computers), etc. Comments are welcome, and I'm going to let the drawing sit awhile to let me "sleep on" the esthetics, and I will write to the (Mandrake) newbie list (I think) to ask for comments on how understandable the drawing is, what might be done to improve it, and whether the colors are reasonable for someone who might be colorblind. (There may be no reasonable solution for someone who is color blind, especially with the limited color selection available in the TWikiDrawPlugin and the problems I've already run into with respect to even reading black text on some of the fill colors (like blue and red), but if I know the problems there is a chance of finding a solution.

• Note to myself: I guess I could put all the legends on a white background within the colored background of an object (something like the way the "or" and "for 24/7 connection" legends are shown on a yellow "island" of color within the pink background of the Email Server background).
• I'd rather edit the drawing myself -- it's getting rather complicated and painful to edit. (However, if you want to learn more about the Plugins.TWikiDrawPlugin and how to use it, see EditingATWikiDrawSketch.)
• I recognize that this sketch is rather busy (and too large for my 800x600 screen). I may maintain the older sketches on EmailServerSketches as simplified versions of this (or I might make new simplified versions from this) -- and maintain this sketch as (one variation) of a complete email handling system.

Contributors

• RandyKramer - 17 Apr 2002
• FaberFedor - vetting and modifications of older versions of sketches, see EmailServerSketches and the revision record for the sketches
• J. Craig Woods ~ 19 Apr 2002
• <If you edit this page, add your name here, move this to the next line>

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