After hearing Baloo's experience,  I started to use mailjet too and very satisfied.
I was wrong to insist to manage the spam reports by myself.
Sometimes you could do nothing to fix it.
One of our users, besides our all explanations, instead of setting her email notifications from site, reported all emails coming from the site as spam to hotmail.
We could never fixed the blockage again in hotmail.
Thank you Baloo for your recommendation, it saved me a lot of headache.