D. J. Bernstein
Internet mail
Internet mail message header format
Miscellaneous: Subject, Comments, Keywords, Encrypted
Subject
Almost all messages have Subject fields.
822 says ``This is intended to provide a summary,
or indicate the nature, of the message.''
For example:
Subject: hope you enjoyed your trip
A message must not have more than one Subject field.
Most MUAs include features to display a one-line summary of each message,
typically including the value of the Subject field,
truncated if necessary.
Excerpt from a new California law (AB 1676), taking effect in 1999:
``In the case of e-mail that consists of unsolicited advertising
material for the lease, sale, rental, gift offer, or other
disposition of any realty, goods, services, or extension of credit,
the subject line of each and every message shall include "ADV:" as
the first four characters.''
When a typical MUA generates a message in
response
to another message,
it copies the Subject field from the original message,
and (for historical reasons)
inserts "Re:" if "Re:"
was not already there:
Subject: Re: hope you enjoyed your trip
The exact handling of Re: depends on the MUA.
For example:
- BSD mail strips all leading whitespace from the original subject.
If the subject starts with "Re:" (without regard to case),
it prints "Subject: " and the subject.
Otherwise it prints "Subject: Re: " and the subject.
Pine behaves the same way.
- elm strips all leading whitespace from the original subject.
If the subject starts with "Re:" (without regard to case),
it strips the "Re:" and any subsequent whitespace.
It then prints "Subject: Re: " and the subject.
- mh strips any number of "Re:"s (without regard to case),
with all surrounding whitespace.
It then prints "Subject: Re: " and the subject.
- netscape strips any number of "Re:"s
(without regard to case),
with all surrounding whitespace,
optionally with a left bracket or parenthesis
followed by any number of digits
followed by a right bracket or parenthesis before the colon.
It then prints "Subject: Re: " and the subject.
- Outlook strips all leading whitespace from the original subject.
If the subject contains a colon as the second, third, or fourth character,
with no previous space, digit, or colon,
Outlook strips everything through the colon,
with all subsequent whitespace.
It then prints "Subject: Re: " and the subject.
Comments
No useful semantics in 822.
Pegasus (before version 2.53) automatically generates a deceptive
Comments: Authenticated sender is <God@heaven.af.mil>
field for each outgoing message.
Keywords
No useful semantics in 822. Used on rare occasions.
Encrypted
No useful semantics in 822.