Failure emails requeu

I am using phplist-3.6.15
when i send campaign, it’s send but it is not ending, the system try to resend to failure emails gain and again until i stop the process manually,
how can i fix this issue

I have found that the emails it is retrying are bac. For me, I looked up the emails ‘still to send’, and after seeing that they were not valid email addresses, I deleted them from the system. That took care of it.