So it is likely that webreus.email server is doing this.
I guess that mailscanner is detecing the phplist user tracking image, thinking that it is suspicious, and replacing it by the its own image, which does not appear to work.
When you have enabled user tracking phplist includes a 1 pixel image similar to this
<img src="https://mysite.com/lists/ut.php?u=6c8baeb98e4e8b59c51b0e52c7b6cbd0&m=123" width="1" height="1" border="0" alt="" />
I think that you need to raise this with the hosting company. Also, if you disable user tracking in config,php does the problem still happen?