D. J. Bernstein
Internet mail
Internet mail message header format

Miscellaneous: Subject, Comments, Keywords, Encrypted


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:


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.


No useful semantics in 822. Used on rare occasions.


No useful semantics in 822.