@bcarney I have received the email now. Looking at the source, the html part looks to be badly formed with some elements before the doctype
Content-Type: text/html; charset=UTF-8
<head>=0A <meta content=3D"text/html;charset=3DUTF-8" http-equiv=3D"=
Content-Type">=0A <meta content=3D"width=3Ddevice-width"/>=0A =
<title></title></head><body><!DOCTYPE html> <html lang=3D"en" xmlns=3D"htt=
p://www.w3.org/1999/xhtml" xmlns:v=3D"urn:schemas-microsoft-com:vml" xmlns:=
o=3D"urn:schemas-microsoft-com:office:office"> <head><!--webversion%%" titl=
Are you editing it with the phplist editor? If so then, because the email is a complete html document, you need to change two settings for the ckeditor plugin on the Settings page
Allow templates to be edited as full HTML pages No
Allow messages to be edited as full HTML pages Yes
otherwise when editing a message some of the html will be removed by the editor.
Or disable the editor and just copy/paste the complete html.
The hosting account that sends the original email successfully is using Exim 4.91. Your email seems to be sent using sendmail
Received: by gop.senate.michigan.gov (Axigen sendmail invoked by uid 33)
id 316FB8; Tue, 28 May 2019 10:57:14 -0400