The line in a Unix mailbox that indicates the start of a new message. In its basic form it consists of the string "From", followed by a space, followed by the message sender and the date, like this:
	From Sat Aug  4 07:17:21 2001

Most software that looks for message boundaries looks for any line that begins with the five characters "From ". Obviously there's a potential ambiguity: what if the body of the message contains a line beginning with the word "From"? This potential problem is worked around in two ways:

  1. A "From " line is only treated as a separator if it's preceded by a blank line (or if it's the first line in the mailbox), and
  2. Whenever a message is appended to the mailbox, if its body happens to contain a blank line followed by a line starting with "From ", that line is altered somehow so it doesn't look like a "From " line. Usually the line is prefixed by a > character, but occasionally a MIME message using quoted-printable encoding has the F in "From" changed to =46. Some mail software is overly cautious, and alters lines beginning with "From " even if they're not preceded by blank lines, or alters lines beginning with "from ", too.

Once upon a time, the "From " line in a Unix mailbox was followed immediately by the body text. (After all, the "From " line contains everything you need to know, right: who it's from and when it arrived. :-) ) Nowadays, the "From " line is followed by the rest of the RFC822 message header, followed by a blank line, followed by the body. All the other lines in the header, of course, begin with a keyword and a colon, which is why I call the separator lines, "From " lines (with a space inside the quotes).

The originating user name given in the "From " line might not match that in the message's main (RFC822) From: line. The From: line is relatively easy to forge, but the "From " line is usually (I think) taken from the SMTP envelope information, which is generally much harder to forge.