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The attached test program shows how parsing an e-mail message with the email package, then converting the resulting message to a string, fails to round-trip properly. Instead it breaks the encoding of the subject line.
The root of the problem: the subject is RFC-2047 quoted, long enough to require line wrapping, and it contains one of the splitchars used by Header.encode() -- meaning a semi-colon or comma. In my example, this is:
Subject: =?utf-8?Q?2010_Foundation_Salary_and_Benefits_Report;_Important_Legislative_Efforts?=
Parsing the message turns that into a string S. generator.Generator._write_headers() then outputs Header(S).encode(), so it keeps treating the value as an ASCII string, and therefore breaks the header at the semicolon, resulting in:
 
Subject: =?utf-8?Q?2010_Foundation_Salary_and_Benefits_Report;<NEWLINE><SPACE>_Important_Legislative_Efforts?=
Newline and space aren't legal in Q encoding, so MUAs give up and display all the =?utf-8?Q? stuff.