When you first set up a dedicated sending domain, the first thing you should do is warm up your new sending domain. Neglecting to do so can result in high bounce rates. Since you are on a new sending domain, inbox providers are keeping a very close eye on the domain to make sure you are a good sender. If you set up a dedicated sending domain and decide to batch and blast (send to your whole list at once), inbox providers will see this as a red flag and bounce the majority of the emails that are being sent to their server. This could also happen even if you sent to an engaged segment with a high recipient count. When you first set up a dedicated sending domain, we would suggest following this guide: Guide to Warming Your Sending Infrastructure to ensure that you properly warm up your new sending domain.