It depends on how “clean” your list is but keep in mind if someone has images turned off and do not display the images, it will not count as an open since the tracking mechanism for opens requires images to be turned on.

Factors that contribute to your open rate include

Company Name and your brand’s reputation with that contact

People open emails from companies and brands they enjoy

Subject line

if it stands out to them or is clearly something they want to know more about they will open

Preheader

If the subject line didn’t catch their attention, maybe something in the preheader will

Time of the year

Depending on your industry certain times of the year may not be as active as others.

Previous content you had delivered that was opened by them

If they opened something in the past but weren’t happy about it, or it just didn’t resonate with them, they may not open more of your messages.

The frequency of messages is too high

If you are sending more often then they anticipated they may decide to stop opening your message.