Parameters for new ticket versus follow-up for incoming mails

[robcar]

I know this is a very basic question, but I didn't manage to find a clear explanation in the manual or in the forum.
So please excuse me if this a FAQ, like it sounds to be.
The question is:
how does OTRS differentiate incoming mails that must create a new ticket from others that should be treated as follow-up to an existing ticket?

I thought ticket number in the subject was the only parameter; and since the system was working fine I didn't bother to have a closer look at the topic.
But today we had an issue when a user forwarded an email that he had sent before with the same subject to the OTRS mailbox (there was no ticket number in the subject) and this was treated as a follow-up to the previous ticket that had been opened with the original mail.

So why, in this case, the system originated a follow-up and not a new ticket?
Secondly, what are the parameters used by OTRS to create follow-ups to previous tickets?

thanks
--
rob

[EXG133]

These settings are in SysConfig -> Core::PostMaster

You may have PostmasterFollowUpSearchInReferences set to yes.

[robcar]

Thanks for the reply.
I have this setting Kernel::System::PostMaster::FollowUpCheck::References set to yes. The description goes like this:

Executes follow-up checks on In-Reply-To or References headers for mails that don't have a ticket number in the subject.

It seems hardcoded, the flag is set, but greyed-out. I guess I should edit the code somewhere.

A thing that I didn't specify in my original post is that the issue we experienced was with a ticket that we had closed. So, rather than tinker with the setting above, I'm wondering if a preferred solution would be to set Follow-up for closed tickets in the queue to reject. I'll experiment a bit.