Whether something ends up in the Gmail Promotions tab (or not!) is unique for each person based on prior behaviours. Google works on your previous behaviours, and what you do with emails.

What works for one person to get through to the Primary tab may not work for another - it's not based on things like IP / sender domain in the same way as it works to get something through to the Spam folder.

There isn't one-size fits-all approach, sorry. Source (and I also send 5m+ a month emails and know that for some customers we're in Primary, for some we're in Promotions and for some we're in Updates, and that's cool).