Hi =?utf-8?q?C=C3=A9dric_Krier?=: (hope the encoding works correcly.)

In message <1603384621.38.0.248728051864.issue2551092@roundup.psfhosted.org>,
=?utf-8?q?C=C3=A9dric_Krier?= writes:
>I do not have anymore the email that was breaking it.
>But from memory it was a subject with some non-ascii char  like "é".

Do you remember if it was a literal utf-8 character or an encoded
entity in the subject. For example in the tests I have:

  Subject: [issue] Test (=?utf-8?b?w4TDlsOc?=) umlauts

and that passes fine. isinstance(word,str) is False as word (under
python3) is a byte b'' not normal string.