Email Deliverability Expert Needed

I need an expert to fix my email deliverability issues. All my emails are landing in Spam, and I need someone with experience in email marketing and spam filtering to resolve this problem. I run a website / platform / marketplace. The website and my inbox has been around for like 2 years. However, I recently did a mass email campaign over a few months via gmail connected to gmass and now all my emails are landing in Spam. I sent over 100,000 emails and a lot of the emails were bad with bounceback etc. So I think I am being viewed as spammer. My domain is hosted with Namecheap and the actual platform website is hosted AWS. When dealing with this, I need to be very careful not to mess this up, as there’s a lot of auto emails that send whenever a user interacts with the platform i.e. welcome emails, notifications etc. These are sent via Sendgrid. I am looking for somebody who is confident they can fix this and get all my emails landing in inbox again. Payment can only be made once this happens.