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ToC3. Using threads

Email messages are prefixed by a number of ‘headers’, like an old-fashioned paper inter-office Memo. As well as the familiar To, From, and Subject that you see on every message, there are headers to let you send messages to groups of people (see section 5.3, ‘Use the right headers’); and dozens of hidden headers that let you navigate through your mailbox and which are also used to direct your email across the Internet.

Most email interfaces, both web-based and stand-alone, can present your email in chronological order, but can also ‘thread’ the messages so that each original message is followed by all the replies, even though they arrived over a span of time along with other messages about different topics.

Contrary to what you might think, the Subject header is not used to organise threads, except by some very poorly written systems, because over time you might have several different conversations with the same subject. Instead, every message is given a Message-ID header with a globally unique ID for your message. And every reply to your message quotes this ID in an In-Reply-To header, and possibly in a References header too.

So when someone replies to an email message, the ID of the original message is used in the reply, and the email programs use this to arrange them to display together, in order, even though they have arrived at different times. This is why it's essential to click  Reply  only when you are actually replying to the subject. If you just want to mail the sender about something else, click  Compose  or  New Message  and of course give it a different subject.

If you click  Reply  are start a new message about something completely different, even if you change the Subject header, the message will end up in their mailbox threaded to the original conversation, and they might miss it.

Equally, if you click  Compose  or  New Message  to send a reply, and laboriously retype the subject, the message will not be attached to the original thread when it arrives in their mailbox. Instead, it will start a new thread of its own, even though it may have the original Subject header.

Getting this wrong (or using an incompetently-designed email program) is probably the biggest single reason that email ‘goes missing’. It hasn't gone missing at all, of course: it's been threaded wrongly because the headers are wrong because the wrong button was clicked.

It is of course possible to see all the headers if you want, especially if you are trying to diagnose a problem and the HelpDesk asks you to send them so they can work out what's happening (see section 5.7, ‘Forwarding messages with all headers intact’).

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, : Key words for use in RFCs to Indicate Requirement Levels (1997).

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