Change an email’s subject when the Topic Changes

On mailing lists, individual messages often spark lively discussions. As these discussions get longer and longer, their topic can change substantially. Often, it has nothing to do with the subject of the original message any more.

This is why you should change the Subject (header line of a message thread) on a mailing list when it becomes apparent that the topic of the thread changes.

Important to note

Be Aware of Thread jacking

Especially on mailing lists, simply changing the Subject  line to start a new conversation can lead to display problems for others (and yourself, too); email programs and services may lump together the wrong messages in threads.

To avoid this problem and practice, also known as "threadjacking", do create a new message with a new Subject: instead of starting with a reply.

Retaining the Original Subject

To make it clear what has happened (that you are continuing an old thread and not starting a new one), you can include the previous Subject line with the new one.

E.g. If the original Subject was "New cloud form discovered!" and you want to change it to "The finest English umbrella" the complete new Subject line could look like "The finest English umbrella (was: New cloud form discovered!)". You can abbreviate the original Subject