TYPO3 Core

if you send a plain text email with t3lib_htmlmail and the default settings and the email contains u URL with an equal sign followed by a number, then email-clients might think it is a quoted printable representation.

to reproduce it:

$mailer = t3lib_div::makeInstance('t3lib_htmlmail');
$mailer->start();
$mailer->subject = 'Test';
$mailer->from_email = 'youremail@foo.bar';
$mailer->setPlain('index.php?id=9');
$mailer->send('youremail@foo.bar');

i open the email in Thunerbird and i see:
index.php?id=
to fix it, you only need to replace all equal signs with =3D

$mailer = t3lib_div::makeInstance('t3lib_htmlmail');
$mailer->start();
$mailer->subject = 'Test';
$mailer->from_email = 'youremail@foo.bar';
$mailer->setPlain(str_replace('=','=3D','index.php?id=9'));
$mailer->send('youremail@foo.bar');

(issue imported from #M15367)