Official comment
You should check with our Postmaster at postmaster@ontraport.com to have them evaluate the drop in open rate you are experiencing! Sometimes it is due to different ways that email service providers like ONTRAPORT will report the stats, and other times it is because your email is now coming from a different server; the Postmaster will have tips on how to get the best open rates possible.
ONTRAPORT only counts "unique" opens of email, which means we record only the first time your contact opens the email. If they open it twice, it still counts as only one "open". Some email service providers will count every open.
The way all of us count the email open is by embedding a single pixel invisible image into the email. The image is fetched from our server, and the URL for the image has unique identifying information to identify the recipient. This is how we will know if "John Smith" opens the email. The flaw in all of this is that some customers will have images turned off in their browsers, so they can open the email and we never know it. This is unlikely to be the issue in your case, as we can expect the same number of your customers to have images turned off now as before you switched. But I wanted to explain how "opens" are counted.
For retargeting purposes, "opens" can be inaccurate because of the above. Sometimes you will see this in the Contact Record, where there is no "open" recorded for an email, but the Contact clicked on a link in the email! ONTRAPORT makes it easy to stack multiple conditions in a Rule so you can check for both "not opened" and "not clicked" to avoid sending email to those Contacts who have images turned off but did open and click on a link in your email. See this blog post for a detailed explanation of opens, clicks, and how to retarget your "non-responders": https://ontraport.com/blog/automation/five-ways-to-resend-an-email
I do want to reiterate that you should reach out to our Postmaster at postmaster@ontraport.com to have him evaluate your response rates and see if we can assist you in getting the most from your email marketing efforts!