I have email deliverability experience. Here is what I can do for you: set up a Postfix mail server in a Ubuntu VPS using an IP without reputation issues, with all required DNS setup (including SPF, DKIM, reverse, etc.) - and make it send the emails using a custom PHP script. I would slow down the amount of emails per minute to avoid being blocked. I will do it only if it is really NOT SPAM. Also, your email contents need to follow "CAN-SPAM" rules and be legal. I will not send any email if it contains virus or anything unfair (phishing, etc.)