More like "almost always". I estimate we average one complaint that turns out
to be message maformation every week or two, and it's been this way for almost
two decades. But despite the fact we as a matter of policy *always* push back
saying "fix the actual problem", I can't recall more than a handful of cases
where this resulted in the actual problem being tracked down and fixed.
Workarounds are a lot easier than, say, getting a printer manufacturer
to update their firmware so it doesn't produce this content-type header: