@bcarney The email sent by phplist is being truncated.
The content type is correct
Content-Type: multipart/alternative;boundary="b1_724d223e149e24be901b583d5154dfd3"
but the email just stops near the end of the plain text part. That is not closed correctly, nor is the html part included.
It looks to be the same problem as reported at If the last character of the footer is a DOT, then DKIM FAILS... WHY?
In that case the cause seemed to be the Exim mail package but I cannot tell whether your server is using that.