New service to avoid getting emails in spam?

Yesterday, after some time without sending any marketing emails for one of my bigger website(around 13k users), I've started again.

But the problem is, after sending around 2k emails, the other ones started to end up on the spam folder for gmail emails, I wasn't 100% sure, but my open rate went from +20% to under 5%, so this is when I've tested it with: https://www.gmass.co/inbox and I was right, the emails started to end up in the spam folder.

So I was thinking about a service to help people avoiding that, by marking the email as not spam, if it ends up in the spam folder, replying to that email(I think engagement helps a lot too), marking it as important, moving it to the main inbox.

So would there be anyone else interested in such a service?