Dan's Mail Format Site:

Headers: Subject

While I was in college, an enterprising student there was selling T-shirts featuring the character he had created in a comic strip in the campus newspaper, with the caption: "Why are we here? What's it all about? Will it be on the test?" Well, there isn't usually an exam coming up where you'll be tested for reading comprehension on your e-mail messages, but you'll probably still find it useful to know the answer to the burning question "What's it all about?" before you open and read each message. An active e-mail user probably gets lots of messages -- some are junk mail to be discarded unread, some are urgent alerts to be dealt with immediately, and others are best off put aside until you've got the spare time to deal with them. The Subject header helps you make these decisions, and also assists in finding the old message you're looking for when delving through your archives of past e-mail.

About the Subject Header

The Subject header is really very simple: it contains free-form text indicating what the message is about. There are no mandatory limitations on its length in the standards (while it's subject to the "soft" 78-character and "hard" 998-character line length limits like anything else in an e-mail message, it can be "folded" into multiple lines), but it's generally expected to keep it reasonably concise; long subjects will probably be truncated upon display.

Coming up with a good subject header is very similar to coming up with a good title for a Web page, an issue I discuss in my Web Tips site. You need something that concisely explains what the content is about, in a non-context-sensitive way (because you can't be sure the reader remembers all the earlier related messages). Put some thought into what to say in the subject of your messages; with the wrong subject, your recipients might ignore your message entirely.

Also, don't write your whole message in the subject line! Some people actually seem to be in the habit of doing this, typing long paragraphs in the subject and sometimes leaving the main body blank. Mail reader programs don't always show extremely long subjects in an easily readable manner, and people aren't accustomed to finding main message bodies in the subject anyway, so your message isn't as likely to be read and understood if you put the body in the subject.

Subjects in Replies

Usually, when you reply to a message, the subject line of the original message is preserved, but preceded with Re: , which is an indicator that the message is a reply to an earlier message. If the original message already has Re: in it, you shouldn't add it again, as Re: Re: Re:... is a rather ugly way to start a subject line. Most mail programs follow this, but there are a few that will stick in multiple "Re:"s... if it happens to you, you should manually trim the subject before sending.

Although the standards explicitly say that "Re:" should be used as the string to mark replies, some mail programs that are configurable to interfaces in different languages will use other strings when non-English languages are used. I'm sure the designers of these programs thought they were doing this out of cultural sensitivity, so as not to "impose" English-language headers on their users. However, the necessity for consistent standards in e-mail headers does require some such impositions -- you can't translate "From", "To", or "Subject" into other languages and expect mail programs to recognize these headers, so they must stay as-is. The fact that they're in English is simply a historical accident caused by the fact that English-speaking people were the ones who built the Internet in the first place. The user interface of a mail program can translate them in their on-screen display, but the actual headers sent over the network must follow the standards, which are in English. Similarly, "Re:" is the standard marker of replies, recognized specifically as such by programs (for instance to avoid adding a second "Re:" to the subject if one is already there), so it should be used as-is regardless of whether it's meaningful in the language used in the message. Anyway, "Re" is not actually in English; it's from the Latin "res", meaning "pertaining to". When programs use variant markers standing for things in different languages, you're likely to end up with a whole string of different prefixes on a subject line of a message that's been forwarded and replied to by several people with mail programs configured for different languages; this, in fact, tends to happen on European mailing lists and newsgroups.

When a sequence of replies goes on for a long time, there's the tendency for what's known as "topic drift," where eventually the subject matter being discussed has little or no relation to what's in the subject line. At some point, it's a good idea for one of the participants to make a manual change to the subject line to bring it in line with the actual subject. This has the problem of disassociating the reply from the message thread from which it arose in reader programs that use subject lines for message arrangement. (If In-Reply-To and Message-ID headers are used, as discussed under Other Headers, the thread structure can still be displayed by mail readers that understand these headers, however.) To ease the transistion, it's common to use a new subject that still contains the old one, like Phases of the Moon (was: Re: Werewolves). This has the disadvantage of keeping an irrelevant subject header around indefinitely as part of the subject line of subsequent messages.

One more note: If you read a mailing list in digest form (with a bunch of messages put together as one message), the subject line is probably something generic like "Digest #123". If you reply to it, you should copy and paste the actual subject of the individual message you're responding to in place of the digest subject... and be sure you trim excess quotage, too, because there's little more wasteful and annoying than attaching an entire quoted digest to a brief response to one message in it.

Subjects in Forwarded Messages

Although the standards don't say anything about this (unlike Re: for replies), it's common for a forwarded message to have the subject line of the original message with Fw: (or sometimes Fwd:) prefixed to it. Like with Re:, it would be a good idea not to keep adding more instances of "Fw:" to the beginning of multiply-forwarded messages, but some programs do anyway.

Other Informational Headers

The standards provide a couple of additional headers for the purpose of describing the content and purpose of a message, though they're less frequently used than the Subject header:

Comments: Used for additional comments about a message. I haven't actually seen a mail program that supports this.
Keywords: Intended for a comma-separated list of keywords, to assist in searching for messages related to a given topic. Pegasus Mail has a configuration option (turned off by default, however) to present a "Keywords" field along with the "Subject" field when a message is being input. I don't know what other mail programs do anything useful with it. If it were widely used, I'm sure spammers would try to abuse it, just like Web spamdexers do the Keywords META tag.

Next: How do mail headers communicate what sorts of non-plaintext things are contained within the message? Not by playing a game of charades, but they do use MIME. Find out how this works in the next article.

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