I had a similar situation this week 11/9/219 for the first time.We have been using PHPlist for years with few problems. I found info here and followed it. Campaign Sent Multiple Times .. (SMTP Error). It worked, but now I am not getting any statistics reported. The problem seems to be that the failures, that are not really failures, keep sending. The info in the link, is a temporary fix. I had upgraded to version 3.4.7 and installed the segmentation plugin and a subscriber plugin. https://resources.phplist.com/plugin/subscribers