Hi Christelle,

This issue has been around in Odoo for quite a while and was very annoying. If you would update your Odoo to the newest codebase those problems should be fixed. I know Odoo changed the way it worked a while back in a pull request (which I can't find directly). Another option is to apply the answer I gave to my own post here: https://www.odoo.com/forum/help-1/question/mass-mailing-campaigns-do-not-track-opened-and-replied-e-mails-64705

I'd suggest a code base update above my fixes though, since that is cleaner. I've attached the patches on my post so you should have everything you need to get it back going!
Note: It is also known that mass mailing tracking will not work with multiple databases. You would either need to have only one database or use a database filter (--db-filter)

Yenthe