ok, we use 0.85.2

perfect, the solution is even better than what I was thinking.

I would try it as soon as I have some time.

Thank you so much.

Another idea would be interesting; If the user adds a follow-up message, the status is automatically changed to active (action required by the technician)

we face similar problems, but for our part, we want a rule that does not rejects the email.
the problem is the following :
1- a user sends an email with people in CC and a ticket is created
2- If a user (who was in CC) answer to the first message, a new ticket is created

we need a rule that compares the title regardless of the presence of the tag [GLPI]. If the rule is a ticket bearing the same title as the email with the same users (email To and CC - user and observer for GLPI), then a follow-up message is created.