Subject: Returned Mail: User Unknown 69 Date: Thu, 13 Sep 90 08:48:06 -0700 From: MAILER-DAEMON@Neon.Stanford.EDU Comment: Redistributed from CS.Stanford.EDU Apparently-To: Juan ECHAGUE e-mail:jve@lifia.imag.fr tel:76 57 46 68 (33) Apparently-To: PS:I’ll summarize if interest,etc.@Neon.Stan- ford.EDU Apparently-To: Juan@Neon.Stanford.EDU Apparently-To: Thanks in advance@Neon.Stanford.EDU Apparently-To: for temporal logics.Comments and references are wel- comed.@Neon.Stanford.EDU Apparently-To: I’m interested in gentzen and natural deduction style axiomatizations@Neon.Stanford.EDU STEP 2: Parse the address. Parsing an electronic mail address is a simple matter of finding the “stan- dard” character that separates the name from the host. Unfortunately, since Unix believes so strongly in standards, it has (at least) three separation characters: “!”, “@”, and “%”. The at-sign (@) is for routing on the Inter- net, the exclamation point (!) (which for some reason Unix weenies insist on calling “bang”) is for routing on UUCP, and percent (%) is just for good measure (for compatibility with early ARPANET mailers). When Joe Smith on machine A wants to send a message to Sue Whitemore on machine B, he might generate a header such as Sue@bar!B%baz!foo.uucp. It’s up to sendmail to parse this nonsense and try to send the message somewhere logical. At times, it’s hard not to have pity on sendmail, since sendmail itself is the victim of multiple Unix “standards.” Of course, sendmail is partially responsible for promulgating the lossage. If sendmail weren’t so willing to turn tricks on the sender’s behalf, maybe users wouldn’t have been so fla- grant in the addresses they compose. Maybe they would demand that their system administrators configure their mailers properly. Maybe netmail would work reliably once again, no matter where you were sending the mail to or receiving it from. Just the same, sometimes sendmail goes too far: Date: Wed, 8 Jul 1992 11:01-0400 From: Judy Anderson yduJ@stony-brook.scrc.symbolics.com To: UNIX-HATERS Subject: Mailer error of the day.

70 Mail I had fun with my own mailer-error-of-the-day recently. Seems I got mail from someone in the “.at” domain. So what did the Unix mailer do with this address when I tried to reply? Why it turned “at” into “@” and then complained about no such host! Or was it invalid address format? I forget, there are so many different ways to lose. …Or perhaps sendmail just thinks that Judy shouldn’t be sending e-mail to Austria. STEP 3: Figure out where it goes. Just as the U.S. Postal Service is willing to deliver John Doe’s mail whether it’s addressed to “John Doe,” “John Q. Doe,” or “J. Doe,” elec- tronic mail systems handle multiple aliases for the same person. Advanced electronic mail systems, such as Carnegie Mellon University’s Andrew System, do this automatically. But sendmail isn’t that smart: it needs to be specifically told that John Doe, John Q. Doe, and J. Doe are actually all the same person. This is done with an alias file, which specifies the mapping from the name in the address to the computer user. Alias files are rather powerful: they can specify that mail sent to a single address be delivered to many different users. Mailing lists are created this way. For example, the name “QUICHE-EATERS” might be mapped to “Anton, Kim, and Bruce.” Sending mail to QUICHE-EATERS then results in mail being dropped into three mailboxes. Aliases files are a natural idea and have been around since the first electronic message was sent. Unfortunately, sendmail is a little unclear on the concept, and its alias file format is a study in misdesign. We’d like to say something insulting, like “it’s from the dark ages of computing,” but we can’t: alias files worked in the dark ages of computing. It is sendmail’s modern, up-to-date alias files that are riddled with problems. Figure 1 shows an excerpt from the send- mail aliases file of someone who maintained systems then and is forced to use sendmail now. Sendmail not only has a hopeless file format for its alias database: many versions commonly in use refuse to deliver mail or perform name resolu- tion, while it is in the processing of compiling its alias file into binary for- mat.