Warming up ips
When there was no email traffic on an ip for about an month, the ip has to be warmed up before sending large amounts of mails with it. That means, starting slowly with some mails to engaged receivers (who opened and clicked mails before). Goal is to get a good reputation at the ISPs.
So you start with a low volume on cold ips, and go higher step by step, so that the ISPs gain trust in you. It takes about 4 to 6 weeks until the ip is ready. Often there is a limit of mails which can be send via a cold ip. If you are a good sender, the limit will increase. I think I don't have to mention that you act with best practise rules in the back of your mind like having DNS records for sending mails and not spaming people. There might be small problems in the send out process, but if you keep your list clean this will be over soon. If you see increasing soft bounce rates during the send out, stop using the ip for this day and start again on the next day.