Structure of Internet Email (CGI Programming with Perl)

9.3. Structure of Internet Email

Email messages are documents containing headers and a body separated by a blank line. Each header contains a field name followed by a colon, some space, and a value. Does this sound familiar? In a basic sense, Internet mail messages are similar in structure to HTTP messages. There are also a number of differences of course: there is no request line or status line; email messages are text documents (binary attachments must be encoded as text before being sent); and most of the field names are different. But if you recall the basic header and body format from our earlier HTTP discussion, that will help you understand how to create email messages.

Some header fields hold email addresses. These can support the full syntax of email addresses that we saw earlier, including the recipient's name in addition to the email address itself, like so:

Mary Smith <mary@somewhere.com>

The shorter mary@somewhere.com is also acceptable.

There are only a few header fields you need to include in email messages: who it is to, who it is from, and what it is about. The first of these fields is actually any of three fields: To , Cc, and Bcc. To and Cc (which stands for carbon-copy) contain the email addresses of any of the recipients of the message. The Bcc field (which stands for blind carbon-copy) does likewise but is deleted from the message before it is sent. The From field contains the email address of the person the message is from. If you want replies to be directed elsewhere, you may also specify that email address in the Reply-To field. Finally, the Subject field contains a summary of the email address.

So far, this is all pretty basic; all of us have received email before. There is, however, a subtle distinction that is important to note. Internet email is in some ways similar to real paper mail: it has a message, which can contain anything, inside an envelope, and the envelope carries the routing information. On formal letters, you often add the recipient's address information to the top of the message; however, it's quite possible to put a message addressed this way in an envelope that is actually addressed and routed to someone else. The same thing is possible with Internet email. The To, Cc, Bcc, and From fields are actually part of the message. They do not determine any of the routing information and do not need to match who the message truly is to or from. You have probably received spam that, according to the To field, appeared to be addressed to someone other than you; likewise, the recipient listed in the From field on most spam is not the true sender. However, for our purposes, we typically do want the address information and these fields to line up. We'll explore this more when we review each of the mailers below.

There are many other important fields that appear in the headers of email addresses, but mailers take care of adding these for you, so we won't include them in our discussion.


9.2. Email Addresses		9.4. sendmail