Here's an actual answer lol. Unfortunately you will probably have to purchase a tool like sendgrid, postmark, mailchimp etc. They keep their reputation high enough to help their clients' deliverability. It sounds like, though, you aren't certain deliverability is an issue, so it may also be a genuine drop in response. Has the email list changed? Has your frequency changed? These are all things to consider. Happy to do a call and try to help.