80 Mail
special header at the beginning of the mail file, Unix assumes that any line
beginning with the letters F-r-o-m followed by a space (“ ”) marks the
beginning of a new mail message.Using bits that might be contained by e-mail messages to represent infor-
mation about e-mail messages is called inband communication, and any-
body who has ever taken a course on telecommunications knows that it is a
bad idea. The reason that inband communication is bad is that the commu-
nication messages themselves sometimes contain these characters. For this
reason, sendmail searches out lines that begin with “From ” and changes
them to “>From.”Now, you might think this is a harmless little behavior, like someone burp-
ing loudly in public. But sometimes those burps get enshrined in public
papers whose text was transmitted using sendmail. The recipient believes
that the message was already proofread by the sender, so it gets printed ver-
batim. Different text preparation systems do different things with the “>”
character. For example, LaTeX turns it into an upside question mark (¿). If
you don't believe us, obtain the paper “Some comments on the assumption-
commitment framework for compositional verification of distributed pro-
grams” by Paritosh Pandya, in “Stepwise Refinement of Distributed Sys-
tems,” Springer-Verlag, Lecture Notes in Computer Science no. 430, pages
622–640. Look at pages 626, 630, and 636—three paragraphs start with a
“From” that is prefixed with a ¿.Sendmail even mangles mail for which it isn’t the “final delivery agent”—
that is, mail destined for some other machine that is just passing through
some system with a sendmail mailer. For example, just about everyone at
Microsoft uses a DOS or Windows program to send and read mail. Yet
internal mail gets goosed with those “>Froms” all over the place. Why?
Because on its hop from one DOS box to another, mail passes through a
Unix-like box and is scarred for life.So what happens when you complain to a vendor of electronic mail ser-
vices (whom you pay good money to) that his machine doesn’t follow pro-
tocol—what happens if it is breaking the law? Jerry Leichter complained to
his vendor and got this response:
Date: Tue, 24 Mar 92 22:59:55 EDT
From: Jerry Leichter <[email protected]>
To: UNIX-HATERS
Subject: That wonderful “>From”From: <A customer service representative>^5