Where did you made your setup ? At smtpboxes or another solution ?
There are various warm-up techniques and pretty much all of them work.
The easiest one is to pick 5% of your sending list volume (for example if your list is 5000 emails, pick-up 250 emails) and send them an intriguing email with the very best practices you have with sending speed of 1 email per 2 minutes.
In time gradually increase the list size.
I setup my own smtp server. I have been reading about some applications that helps with engagement using you own group of accounts. I hear they're very effective when it comes to building reputation.
I have 5 ips and 5 domains. My goal is to be able to send at least 500k/day per ip once they are warmed up.