Discovered while writing tests for MDL-48002.
Clients which use a quoted-printable encoding will encode single quotes as their HTML entity version:

'

However, the version of html2text that we use only converts double quotes:
https://github.com/moodle/moodle/blob/master/lib/html2text.php#L522

        $text = html_entity_decode($text, ENT_COMPAT, 'UTF-8');

Unfortunately, MDL-49527 was rejected and we did not update HTML2Text yet. The version in master for roundcube has changed that line (https://github.com/roundcube/roundcubemail/blob/master/program/lib/Roundcube/rcube_html2text.php#L473):

        $text = html_entity_decode($text, ENT_QUOTES, $this->charset);