Inbox delivery depends on multiple factors.
ISPs such as (i.e. Gmail, Yahoo etc) will consider many things for your Inbox delivery such as your domain/IPs/server reputation, your email content, your email list, sending volumes per hour/ per day towards their user’s email IDs, Users reactions towards your emails etc.
We also seen that for different users it works differently means if for one user it will delivery into the Inbox, so for other users it could be delivered to promotion or spam folder.
It will totally depend on that particular user’s past behaviors towards such emails) ISPs will react based on users behaviors as well.
Initially, it could have happened that your email will delivery to the spam folder since your domain/server/IP will be a new setup and ISPs will take time to recognize your domain/IPs as an email sender.
You have to follow IP warm up the process in order to get good delivery with a new setup server. So after warm up, you can reach to ISP’s Inbox but at the same the time you also have to work on your email content and contact list.Our customer is getting from 5 to 80% open-rates (we can measure the delivery based on the number of open/read emails ).
This variation in delivery rate is completely based on the above-mentioned factors.