Flask-Mail failed to send mails with specific subjects

[ArchibaldArt]

For the reason of the unresolved Python issue http://bugs.python.org/issue27240
mail sending by Flask-Mail failed for some mail Subjects.
I.e. for subject: "i demandé comment il avait fait pour éliminer de la sorte trois adversaires dans un si petit espace et lui-même na pas su me dire."

Flask_mail makes different calls in method Message.as_bytes() for Python v < 3.4 and >= 3.4:

def as_bytes(self):
    if PY34:
        return self._message().as_bytes()
    else: # fallback for old Python (3) versions
        return self._message().as_string().encode(self.charset or 'utf-8')

I've taken above error only for PY34 branch (_message().as_bytes() called) and no error for old python branch (_message().as_string() called).

Temporary I've patched my flask_mail Message.as_bytes() to run always as_string().encode() for all Python versions (I've tested on 3.5.2). It is not guaranteed solution, but it is the one solution I've found to work.

[peteristhegreat]

It looks like subject lines must be in ascii with Flask Mail. Accent characters and other UTF-8 or UTF-16 characters throw it off. Thanks for posting this.

[davidism]

Minimum supported version is now Python 3.8. I can't reproduce this.