FWIW in the HTTP bug <https://bugs.python.org/issue24363#msg244676>, David said “when seeing a line that doesn't look like a header the error recovery is to treat that line as the beginning of the body (ie: assume the blank line is missing).” I have no experience with email and RFC 5322 header handling, but it does make more sense to me to handle this as a defect in the header section, _not_ a genuine transition to the body (same as desired for the HTTP case).

Here is a patch that revives MalformedHeaderDefect (see Issue 14925), and continues parsing the rest of the header section instead of starting the body. But I am not sure how safe this change is. I did have to fix one unrelated set of tests (see headertest_msg in the Test8BitBytesHandling class) that did not include a blank line and was relying on the old behaviour.