You've listed the first thing that gets checked. I found a solution to the problem, which I wrote about in the message above and provided a link to the post with the solution.
Configuration / Mail Proxy / Options - Accept Broken MIME Structure -> Yes
Christian Kraus reacted to Dep1911's post in the thread Error : said: 451 4.4.0 detected undelivered mail to <address> with
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You've listed the first thing that gets checked. I found a solution to the problem, which I wrote about in the message above and provided a link to the post with the solution. Configuration / Mail Proxy / Options - Accept Broken MIME Structure -> Yes