Won’t sent mails, unexplained errors

Installation and setup are quick and easy. The integration with wpforms didn’t work well enough so I had to use the integrated form tool. I’m using the SMTP option to sent my mails, and the test mail was sent successfully. But when it came to sent mails to a test group of subscribers, it failed. Some were marked as “failed” wihtout an explanation as to why and others are still being sent – aparrently no time out or anything like that.