E-mail thread identification

How do e-mail clients know whether or not an e-mail is part of a thread? (For example, Thunderbird can fold e-mails into threads.) Do they just guess based on the Subject line, or do they know for certain based on an e-mail field?

I was planning to tackle some more advanced filter programming on my Postfix server.

It's in the headers of the email message

Its the email subject,
If you modify the subject its gone out of that thread.

When you look at the plain email file, you will find entries like:

Message-ID:
References:
In-Reply-To:

with some unique values behind which will be used. If the email client can’t use it for threads, it will sometimes use the subject.

Correct, as above