Hello,
Recently, I found this topic here https://woofresh.com/question/how-can-i-maximize-the-open-rate-of-my-emails/  and I will tell you exactly what I said there:
I think the main factors that influence the open rate are:
Check email list: 

Get permission: Use double opt-in  that requires new subscribers to confirm their signup, … (more)

Hello,
I think the main factors that influence the open rate are:
Check email list: 

Get permission: Use double opt-in  that requires new subscribers to confirm their signup, so you know they want to receive emails from you.
Include an update profile link in campaigns … (more)