I did a mass mailing test and perform an unsubscribe test from my mailbox without connecting to my Odoo server. A 403 error appears and says sue I have no rights. When I log in with a portal account, same thing.
How to unregister mass mailing on Odoo 12 ? pls
I found why. In the mailing, I clicked on "Posted". And I saw that the email will be sent in one hour. Presumed as I am to tested the unsubscription of a mocked. I clicked on "test" to send this email to a subscriber. Then in my test mailbox, I went on the mail and clicked on "unsubscribe" and the message 403 appeared on my browser. I did some research, I did not understand anything on the forums that I went. An hour later, my test mailbox received the planned email. So, I go on this new mail, I click on unsubscription and I can finally access the unsubscribe page.
To test if unsubscribing is working properly, wait for odoo to send your mailings.