Most likely you have deliverability problem, check SMTP provider, their IPs reputations, ...
I had the same issue, I bought an SMTP from hostlyt.com, expected to deliver 100k per day,
but when I start testing, I found all the IPs are blacklisted, and the server not delivering anywhere, just few gmail went 100% straight to spam folder.
I had the same issue, I bought an SMTP from hostlyt.com, expected to deliver 100k per day,
but when I start testing, I found all the IPs are blacklisted, and the server not delivering anywhere, just few gmail went 100% straight to spam folder.