If you have a new email server you have to do domain warmup. That basically entails sending out emails to show ESPs like Google that they aren't spam. I did thi

If you have a new email server you have to do domain warmup. That basically entails sending out emails to show ESPs like Google that they aren't spam. I did this at a previous job, we would have email lists of millions of people and have a monthly plan where the first day we send emails to 1000 users, second day 2000, until at the end of the month we send to all. We'd get blacklisted sometimes and then have to apply to be whitelisted. It is not easy.