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 depend on that particular user’s past behaviors towards such emails too (ISPs will react based on users behaviors as well)
Initially, your IPs/domains reputation is the neutral state and you can gain positive reputation via your mailing. (With good and hygiene email list, good email content and with users engagement)
ISPs will take some time to recognize you as a genuine sender and for that, you need to you have to follow the IP warm-up process in order to get good delivery with a new setup server.
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.
These are some real-time example as mentioned below.
Statistics Report 1:
Statistics Report 2:
Statistics Report 3:
Statistics Report 4: