Buttondown Documentation
We know you want every newsletter email you send to be opened and read by all of your subscribers. Emails that show up in the "Primary" tab in Gmail are much more likely to be read than ones that get delivered to secondary tabs, like the "Promotions" tab.
The "Promotions" tab is not the enemy. It is your subscribers' friend. By filtering low-value or sales-focused emails into the "Promotions" tab, Gmail is making an effort to make reading email better.
It's your job to make your newsletter into something your subscribers really want to read. You can help your email show up in the right place by using the same best practices you use to make sure it doesn't end up in the spam folder:
Unfortunately, there is not a foolproof method that guarantees your newsletter will always appear in the "Primary" tab. There isn't one: Gmail's categorization schemes are deliberately vague. The best way to land in your subscribers' "Primary" inbox is to produce great content.
You can read more about avoiding the Promotions tab on SalesHandy.