Hi @forman 

There is no strict rule about this since if google tell us why and how they do this we will all find an easy workaround. The main thing that is known is that if you have $ sign or some words that are widely used for sales in the subject lines combined with low open rate of previous emails gmail will flag them as promotional emails. 
Other thing is the open rates are closely connected to gmail promotions tab, the lower open rate the bigger the chances it will go to promotional email and the more your emails go to promotional there are less chances they will get opened.

My suggestion is try with simple subject lines that don’t talk about sales in any way, and than for a few weeks only send to your highly engaged segment. Rise your open rate to at least 20-25% if possible. With this klaviyo will put you into IP with accounts with higher open rate and also gmail will start seeing your emails as less spammy.

Once your ratings get better and you start seeing all your emails in inbox you can start sending emails to bigger list. Just make sure not to move to fast because if the lists are bad and with low open rates you will get back to promotions fast.

I also recommend often cleaning of you inactive subscribers. If someone received 10 emails and didn’t open even 1, there is no point of keeping them around, you wont make more sales by sending emails to people who never open them.

Thanks for your magic words!

So, I understand that the main thing is the subject line.

@forman you are sending marketing emails so in most cases you will land in the Promo tab which is totally fine. You main goal should be to avoid the spam folder.

Also you can try text based emails only. They have more chances to land in the main tab vs promo tab.