LOCALHOSTRELAY, IPv6 localhost - ::1

The subject of the email is like the subject and this are the first lines, followed by a list of emails: Time: Fri Nov 16 19:53:07 2018 +0000 Type: LOCALHOSTRELAY, IPv6 localhost - ::1 Count: 180 emails relayed Blocked: No Sample of the first 10 emails: I have the same issue that I saw here in other posts but the posts were old and the forum system didn't allowed me to post a reply so I had to make a new one. I didn't found an answer for what I need. It is not SPAM. And the user was not compromised. And, yes, they should use a newsletter software for this things but this is not always possible or feasible for them. Sometimes they just need to send an email to a specific bunch of people. It's an NGO. Are the emails not being sent? How can I deactivate this for some users until I convince them on using something better and have the time to implement it? Thanks in advance, Lopo