New SMTP Server

I have been building a new SMTP Server on the dedicated cloud server in order to have a unique outbound email address. However, since creating this, we have noticed THOUSANDS of messages that never went through before have appeared. Even ones from up to 3 years ago. My question is, did changing the HOSTNAME for the environment cause phpList to fire off and send all the old jobs for some reason or should I assume these were messages held due to DKIM and SPF errors and are now being released since this IP now has a “REAL” SMTP Server associated with it?

The tests show that these were all held by email servers that refused to send them because they failed either dmarc, dkim or spf. Things have finally quieted down and are working well now. I have learned a lot about setting up a new SMTP server so if you have questions, I will gladly share what I learned.