Dear Vincent,
I think that your problem resides in the subject of your emails and the body. If Google detects specific words in the content, they filter it as offers. Email filters today are a pain in the back, so my suggestion is to find a way to write your emails in a more personal way.
Discounts or offers buy now or sale, all those keywords will put the email in the promo tab. My advice? Use different words when you are crafting your emails, bro.
That's my own solution. Don't get too technical with dedicated IP's. Just change your writing style.
Good luck,
Vlad
thanks a lot!!
do you think this problem would affect also the domain name?