This is it! Super advice and sharing experience. I am quite happy now :) as a temporary solution, I changed the "from" to catchall@.. so all subs get the notification. At the same time I will work on your advice.

Well, after many many many conversations with Mailjet, they told me that I had to ask them to set a "default sender email". At the end they setup the mail account I told them to set as a default sender and this was a solid solution. After a month, they made another problem which is still going on. Odoo sends email with uuid in the header and during the return of the email (client to odoo), mailjet removes the uuid from the header :( I complained about this via LinkeIn so I am hopping they will solve this problem. Honestly try to ask to set up as a default sender but I can't guarantee that Mailjet is a solid company to use. I hope I was helpful to you.